introduce async_callback_ftype

introduce async_callback_ftype

This introduces async_callback_ftype.  This is needed for
make-target-delegates to work properly, as it doesn't implement a real
parser.  I think it's also an ok cleanup in its own right.

2014-01-15  Tom Tromey  <tromey@redhat.com>

	* target.h (async_callback_ftype): New typedef.
	(struct target_ops) <to_async>: Use it.