Update visibility map in the second phase of vacuum.

There's a high chance that a page becomes all-visible when the second phase
of vacuum removes all the dead tuples on it, so it makes sense to check for
that. Otherwise the visibility map won't get updated until the next vacuum.

Pavan Deolasee, reviewed by Jeff Janes.
Fix bogus when-to-deregister-from-listener-array logic.

Since a backend adds itself to the global listener array during
Exec_ListenPreCommit, it's inappropriate for it to remove itself during
Exec_UnlistenCommit or Exec_UnlistenAllCommit --- that leads to failure
when committing a transaction that did UNLISTEN then LISTEN, since we end
up not registered though we should be.  (This leads to missing later
notifications, or to Assert failures in assert-enabled builds.)  Instead
deal with deregistering at the bottom of AtCommit_Notify, when we know the
final state of the listenChannels list.

Also, simplify the representation of registration status by replacing the
transient backendHasExecutedInitialListen flag with an amRegisteredListener
flag.

Per report from Greg Sabino Mullane.  Back-patch to 9.0, where the problem
was introduced during the LISTEN/NOTIFY rewrite.
Fix contrib/pg_trgm's similarity() function for trigram-free strings.

Cases such as similarity('', '') produced a NaN result due to computing
0/0.  Per discussion, make it return zero instead.

This appears to be the basic cause of bug #7867 from Michele Baravalle,
although it remains unclear why her installation doesn't think Cyrillic
letters are letters.

Back-patch to all active branches.
Fix CVE-2013-0255 properly.

Revert commit ab0f7b6089fd215f6ce6081e2e222c38d643a526 (in HEAD only)
in favor of the proper solution, which is to declare enum_recv() correctly
in the system catalogs.  It should be declared to take type "internal"
not "cstring".

Also improve the type_sanity regression test, which should have caught
this typo, so that it actually would.  Most of the relevant checks on
the signature of type I/O functions should not have been restricted to
basetypes/pseudotypes, as they should apply to any type's I/O functions.