rtpoll: Separate out before/after/work callback userdata

rtpoll: Separate out before/after/work callback userdata

It is possible that we might want to have a separate userdata to be used
for these callbacks, so let's split them out.

This is particularly needed when using an pa_rtpoll_item around pa_fdsem
since that uses its own before/after callback but will essentially have
whatever is using the fdsem set up the work callback appropriately (and
thus at least the work callback's userdata needs to be separated from
the before/after callback -- we might as well then just separate all
three).

Signed-off-by: Arun Raghavan <arun@arunraghavan.net>