remote.c/all-stop: Implement TARGET_WAITKIND_NO_RESUMED and TARGET_WNOHANG

remote.c/all-stop: Implement TARGET_WAITKIND_NO_RESUMED and TARGET_WNOHANG

Even though "target remote" supports target-async, the all-stop
target_wait implementation ignores TARGET_WNOHANG.  If the core
happens to poll for events and we've already read the stop reply out
of the serial/socket, remote_wait_as hangs forever instead of
returning an indication that there are no events to process.  This
can't happen currently, but later changes will trigger this.

gdb/ChangeLog:
2015-08-07  Pedro Alves  <palves@redhat.com>

	* remote.c (remote_wait_as): If not waiting for a stop reply,
	return TARGET_WAITKIND_NO_RESUMED.  If TARGET_WNOHANG is
	requested, don't block waiting forever.