Extended-remote catch exec

Extended-remote catch exec

This patch implements exec catchpoints for extended-remote Linux
targets.  The implementation follows the same approach used for
fork catchpoints, implementing extended-remote target routines for
inserting and removing the catchpoints by just checking if exec events
are supported.  Existing host-side code and previous support for
extended-remote exec events takes care of the rest.

gdb/ChangeLog:

	* remote.c (remote_exec_event_p): New function.
	(remote_insert_exec_catchpoint): New function.
	(remote_remove_exec_catchpoint): New function.
	(init_extended_remote_ops): Initialize extended_remote_ops
	members to_insert_exec_catchpoint and
	to_remove_exec_catchpoint.