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/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)
   --