waiter: Don't rely on global variables to keep waiter state

waiter: Don't rely on global variables to keep waiter state

Rather than defining the set of waiters (and pollfds) in waiter.c, add a
struct waitset to contain these.  A waitset is created with
waitset_create, which is passed to the waiter_* functions.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Geoff Levand <geoff@infradead.org>