Introduce target_{stop,continue}_ptid

Introduce target_{stop,continue}_ptid

This commit introduces two new functions to stop and restart target
processes that shared code can use and that clients must implement.
It also changes some shared code to use these functions.

gdb/ChangeLog:

	* target/target.h (target_stop_ptid, target_continue_ptid):
	Declare.
	* target.c (target_stop_ptid, target_continue_ptid): New
	functions.
	* common/agent.c [!GDBSERVER]: Don't include infrun.h.
	(agent_run_command): Always use target_stop_ptid and
	target_continue_ptid.

gdb/gdbserver/ChangeLog:

	* target.c (target_stop_ptid, target_continue_ptid): New
	functions.