Fix up some target is-async vs can-async confusions

Fix up some target is-async vs can-async confusions

In all these cases we're interested in whether the target is currently
async, with its event sources installed in the event loop, not whether
it can async if needed.  Also, I'm not seeing the point of the
target_async call from within linux_nat_wait.  That's normally done on
resume instead, which this target already does.

Tested on x86_64 Fedora 20, native and gdbserver.

gdb/
2015-02-03  Pedro Alves  <palves@redhat.com>

	* linux-nat.c (linux_child_follow_fork, linux_nat_wait_1): Use
	target_is_async_p instead of target_can_async.
	(linux_nat_wait): Use target_is_async_p instead of
	target_can_async.  Don't enable async here.
	* remote.c (interrupt_query, remote_wait, putpkt_binary): Use
	target_is_async_p instead of target_can_async.