lib: Enhance poll_funcs_tevent for multiple tevent_contexts

lib: Enhance poll_funcs_tevent for multiple tevent_contexts

With this patch it will be possible to use nested event contexts with
messaging_filtered_read_send/recv. Before this patchset only the one and only
event context a messaging_context is initialized with is able to receive
datagrams from the unix domain socket. So if you want to code a synchronous
RPC-like operation using a nested event context, you will not see the reply,
because the nested event context does not have the required tevent_fd's.
Unfortunately, this patchset has to add some advanced array voodoo. The idea
is that state->watches[] contains what we hand out with watch_new, and
state->contexts contains references to the tevent_contexts. For every watch we
need a tevent_fd in every event context, and the routines make sure that the
arrays are properly maintained.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>