Delete the gdb-inferior pty when the gdb process exits.

Delete the gdb-inferior pty when the gdb process exits.

* lisp/progmodes/gdb-mi.el (gdb-inferior-io--maybe-delete-pty): New
function to call delete-process on the gdb-inferior buffer's pty.
(gdb-reset): Use it, instead of relying on kill-buffer to kill the
pty process.
(gdb-update): New arg to suppress talking to the gdb process.
(gdb-done-or-error): Use it.
(gdb-stopped-functions): Rename from gdb-stopped-hooks.
(gdb): Call gdb-inferior-io--maybe-delete-pty as a workaround for
sentinel not being called.

* lisp/comint.el (make-comint-in-buffer, comint-exec): Doc fix.