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/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                                             |