term: use an argument vector for termopen().

Old behaviour: termopen('cmd') would run `&shell &shcf "cmd"`, which
caused the functional tests to fail on some systems due to the process
not "owning" the terminal. Also, it is inconsistent with jobstart().

Modify termopen() so that &shell is not invoked, but maintain the old
behaviour with :terminal. Factor the common code for building the
argument vector from jobstart() and modify the functional tests to call
termopen() instead of :terminal (fixes #2354).

Also:
 * Add a 'name' option for termopen() so that `:terminal {cmd}` produces
   a buffer named "term//{cwd}/{cmd}" and termopen() users can customize
   the name.
 * Update the documentation.
 * Add functional tests for `:terminal` sinse its behaviour now differs
   from termopen(). Add "test/functional/fixtures/shell-test.c" and move
   "test/functional/job/tty-test.c" there, too.

Helped-by: Justin M. Keyes <@justinmk>

test/functional/terminal/cursor_spec.lua

@@ -144,7 +144,7 @@ describe('cursor with customized highlighting', function()
       [6] = {foreground = 130},
     })
     screen:attach(false)
-    execute('term "' ..nvim_dir.. '/tty-test"')
+    execute('call termopen(["'..nvim_dir..'/tty-test"]) | startinsert')
   end)
 
   it('overrides the default highlighting', function()

test/functional/terminal/ex_terminal_spec.lua

@@ -0,0 +1,61 @@
+local helpers = require('test.functional.helpers')
+local Screen = require('test.functional.ui.screen')
+local clear, wait, nvim = helpers.clear, helpers.wait, helpers.nvim
+local nvim_dir = helpers.nvim_dir
+local execute, source = helpers.execute, helpers.source
+local eq, neq = helpers.eq, helpers.neq
+
+describe(':terminal', function()
+  local screen
+
+  before_each(function()
+    clear()
+    screen = Screen.new(50, 7)
+    screen:attach(false)
+    nvim('set_option', 'shell', nvim_dir..'/shell-test')
+    nvim('set_option', 'shellcmdflag', 'EXE')
+
+  end)
+
+  it('with no argument, acts like termopen()', function()
+    execute('terminal')
+    wait()
+    screen:expect([[
+      ready $                                           |
+      [Program exited, press any key to close]          |
+                                                        |
+                                                        |
+                                                        |
+                                                        |
+      -- TERMINAL --                                    |
+    ]])
+  end)
+
+  it('executes a given command through the shell', function()
+    execute('terminal echo hi')
+    wait()
+    screen:expect([[
+      ready $ echo hi                                   |
+                                                        |
+      [Program exited, press any key to close]          |
+                                                        |
+                                                        |
+                                                        |
+      -- TERMINAL --                                    |
+    ]])
+  end)
+
+  it('allows quotes and slashes', function()
+    execute([[terminal echo 'hello' \ "world"]])
+    wait()
+    screen:expect([[
+      ready $ echo 'hello' \ "world"                    |
+                                                        |
+      [Program exited, press any key to close]          |
+                                                        |
+                                                        |
+                                                        |
+      -- TERMINAL --                                    |
+    ]])
+  end)
+end)

test/functional/terminal/helpers.lua

@@ -56,7 +56,7 @@ local function screen_setup(extra_height)
   -- tty-test puts the terminal into raw mode and echoes all input. tests are
   -- done by feeding it with terminfo codes to control the display and
   -- verifying output with screen:expect.
-  execute('term ' ..nvim_dir.. '/tty-test')
+  execute('enew | call termopen(["'..nvim_dir..'/tty-test"]) | startinsert')
   -- wait for "tty ready" to be printed before each test or the terminal may
   -- still be in canonical mode(will echo characters for example)
   --

test/functional/terminal/highlight_spec.lua

@@ -27,7 +27,7 @@ describe('terminal window highlighting', function()
       [8] = {background = 11}
     })
     screen:attach(false)
-    execute('term "' ..nvim_dir.. '/tty-test"')
+    execute('enew | call termopen(["'..nvim_dir..'/tty-test"]) | startinsert')
     screen:expect([[
       tty ready                                         |
                                                         |
@@ -133,7 +133,7 @@ describe('terminal window highlighting with custom palette', function()
     })
     screen:attach(true)
     nvim('set_var', 'terminal_color_3', '#123456')
-    execute('term "' ..nvim_dir.. '/tty-test"')
+    execute('enew | call termopen(["'..nvim_dir..'/tty-test"]) | startinsert')
     screen:expect([[
       tty ready                                         |
                                                         |

test/functional/terminal/scrollback_spec.lua

@@ -332,7 +332,7 @@ describe('terminal prints more lines than the screen height and exits', function
     clear()
     local screen = Screen.new(50, 7)
     screen:attach(false)
-    execute('term ' ..nvim_dir.. '/tty-test 10')
+    execute('call termopen(["'..nvim_dir..'/tty-test", "10"]) | startinsert')
     wait()
     screen:expect([[
       line6                                             |