Separate out ANSI-standard signals

This commit reorders various pieces of code to separate ANSI-standard
signals from other signals that need checking.  Comments are added to
document this, and to document the ordering of the signals.

gdb/
2014-06-09  Gary Benson  <gbenson@redhat.com>

	* common/signals.c (gdb_signal_from_host): Reorder to separate
	the always-available ANSI-standard signals from the signals that
	require checking.
	(do_gdb_signal_to_host): Likewise.
	* proc-events.c (signal_table): Likewise.

gdb/testsuite/
2014-06-09  Gary Benson  <gbenson@redhat.com>

	* gdb.base/sigall.c [Functions to send signals]: Reorder to
	separate the always-available ANSI-standard signals from the
	signals that require checking.
	(main): Likewise.
	* gdb.reverse/sigall-reverse.c [Functions to send signals]:
	Likewise.
	(main): Likewise.