Simplify allocations and fix merge and continue actions [BZ #28931]

Simplify allocations and fix merge and continue actions [BZ #28931]

Allocations for address tuples is currently a bit confusing because of
the pointer chasing through PAT, making it hard to observe the sequence
in which allocations have been made.  Narrow scope of the pointer
chasing through PAT so that it is only used where necessary.

This also tightens actions behaviour with the hosts database in
getaddrinfo to comply with the manual text.  The "continue" action
discards previous results and the "merge" action results in an immedate
lookup failure.  Consequently, chaining of allocations across modules is
no longer necessary, thus opening up cleanup opportunities.

A test has been added that checks some combinations to ensure that they
work correctly.

Resolves: BZ #28931

Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
Reviewed-by: DJ Delorie <dj@redhat.com>