Extended-remote exec documentation

Extended-remote exec documentation

This patch adds documentation of support for exec events on
extended-remote Linux targets.

gdb/ChangeLog:

	* NEWS: Announce new remote packets for the exec-events
	feature and the exec-events feature and associated commands.

gdb/doc/ChangeLog:

	* gdb.texinfo (Remote Configuration): Add exec event
	feature to table of packet settings.
	(Stop Reply Packets): Add exec events to the list of stop
	reasons.
	(General Query Packets): Add exec events to tables of
	'gdbfeatures' and 'stub features' supported in the qSupported
	packet, as well as to the list containing stub feature
	details.