Handle correctly passing a bad interpreter name to new-ui

Handle correctly passing a bad interpreter name to new-ui

When a bad interpreter name is passed to new-ui, such as:

  (gdb)  new-ui bloop /dev/pts/10

A partially created UI is left in the UI list, with interp set to NULL.
Trying to do anything that will print on this UI (such as "start") will
cause a segmentation fault.

Changes in v2:

  - Use with_test_prefix to namespace test procedures
  - Give an explicit stable test name
  - Add a "bad terminal path" test
  - Remove useless runto_main
  - Add missing intro comments

I did not factor out the pty spawn, as there is some magic involved I
don't quite understand.  But it wouldn't bring that much anyway.

gdb/ChangeLog:

	* top.h (make_delete_ui_cleanup): New declaration.
	* top.c (delete_ui_cleanup): New function.
	(make_delete_ui_cleanup): New function.
	(new_ui_command): Create restore_ui cleanup earlier, create a
	delete_ui cleanup and discard it on success.

gdb/testsuite/ChangeLog:

	* gdb.base/new-ui.exp (do_test_invalid_args): New
	procedure.