Share some inferior-related Windows code

Share some inferior-related Windows code

This adds a couple of functions to nat/windows-nat.c and changes gdb
and gdbserver to use them.  One function checks the list of pending
stops for a match (not yet used by gdbserver, but will be in a
subsequent patch); and the other is a wrapper for ContinueDebugEvent
that always uses the last "real" stop event.

gdb/ChangeLog
2020-04-08  Tom Tromey  <tromey@adacore.com>

	* windows-nat.c (windows_continue): Use matching_pending_stop and
	continue_last_debug_event.
	* nat/windows-nat.h (matching_pending_stop)
	(continue_last_debug_event): Declare.
	* nat/windows-nat.c (DEBUG_EVENTS): New define.
	(matching_pending_stop, continue_last_debug_event): New
	functions.

gdbserver/ChangeLog
2020-04-08  Tom Tromey  <tromey@adacore.com>

	* win32-low.c (child_continue): Call continue_last_debug_event.