linux-nat.c: clean up pending status checking and resuming LWPs

Whenever we resume an LWP, we must clear a few flags and flush the
LWP's register cache.  We actually currently flush the register cache
of all LWPs, but that's unnecessary.  This patch makes us flush the
register cache of only the LWP that is resumed.  Instead of open
coding all that in many places, we use a helper function.

Likewise, we have two fields in the LWP structure where a pending
status may be recorded.  Add a helper predicate that checks both and
use it throughout instead of open coding the checks.

gdb/
2015-01-09  Pedro Alves  <palves@redhat.com>

	* linux-nat.c (linux_resume_one_lwp): New function.
	(resume_lwp): Use lwp_status_pending_p and linux_resume_one_lwp.
	(linux_nat_resume): Use lwp_status_pending_p and
	linux_resume_one_lwp.
	(linux_handle_syscall_trap): Use linux_resume_one_lwp.
	(linux_handle_extended_wait): Use linux_resume_one_lwp.
	(status_callback, running_callback): Use lwp_status_pending_p.
	(lwp_status_pending_p): New function.
	(stop_and_resume_callback): Use lwp_status_pending_p.
	(linux_nat_filter_event): Use linux_resume_one_lwp.
	(linux_nat_wait_1): Always use status_callback to look for an LWP
	with a pending status.  Use linux_resume_one_lwp.
	(resume_stopped_resumed_lwps): Use lwp_status_pending_p and
	linux_resume_one_lwp.