Rename target_{stop,continue}_ptid

Rename target_{stop,continue}_ptid

This commit renames target_stop_ptid as target_stop_and_wait and
target_continue_ptid as target_continue_no_signal.  Comments are
updated to more fully describe the functions' behaviour.

gdb/ChangeLog:

	* target/target.h (target_stop_ptid): Renamed as...
	(target_stop_and_wait): New function.  Updated comment.
	All uses updated.
	(target_continue_ptid): Renamed as...
	(target_continue_no_signal): New function.  Updated comment.
	All uses updated.