Hurd: In the CLI, use parse_thread_id instead of global_thread_id_to_ptid

Hurd: In the CLI, use parse_thread_id instead of global_thread_id_to_ptid

Follow-up to commit 14f6890677849172a4b13779acd9089c9baa3a81.
global_thread_id_to_ptid expects global thread numbers, which are nowadays only
used in MI, never presented to the user in the CLI.  Since this is a CLI
command, it should accept the inferior-qualified format instead.

	gdb/
	* gnu-nat.c (set_sig_thread_cmd): Use parse_thread_id instead of
	global_thread_id_to_ptid.