Debug output tweaks in the Linux target backends

Debug output tweaks in the Linux target backends

This adds/tweaks a few debug logs I found useful recently.

gdb/gdbserver/ChangeLog:
2015-03-24  Pedro Alves  <palves@redhat.com>

	* linux-low.c (check_stopped_by_breakpoint): Tweak debug log
	output.  Also dump TRAP_TRACE.
	(linux_low_filter_event): In debug output, distinguish a
	resume_stop SIGSTOP from a delayed SIGSTOP.

gdb/ChangeLog:
2015-03-24  Pedro Alves  <palves@redhat.com>

	* linux-nat.c (linux_nat_resume): Output debug logs before trying
	to resume the event lwp.  Use the lwp's ptid instead of the passed
	in (maybe wildcard) ptid.
	(stop_wait_callback): Tweak debug log output.
	(check_stopped_by_breakpoint): Tweak debug log output.  Also dump
	TRAP_TRACE.
	(linux_nat_filter_event): In debug output, distinguish a
	resume_stop SIGSTOP from a delayed SIGSTOP.  Output debug logs
	before trying to resume the lwp.