The abort came from using libuv tty handle on non-tty fd. Use uv_pipe_t in these
cases. Also add simple test for this case.

Target: make all tests run with chmod -x ~/.config/nvim ~/.local/share/nvim.

The tests use `termopen` to spawn nvim and verify the TUI.

Close #3332

Since close_cb may free the terminal structure, save the "wipe" flag before
calling it.

Previously, the screen test was expecting the screen state to be
identical to the previous screen test in `thelpers.screen_setup()`,
which is indeterministic. (The later screen test can accidentally
still see the previous identical state). The solution is to add a test
for a intermediate different state.

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>