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/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)