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>