Fix lexing of U& sequences just before EOF.

Commit a5ff502fceadc7c203b0d7a11b45c73f1b421f69 was a brick shy of a load
in the backend lexer too, not just psql.  Per further testing of bug #9068.

In passing, improve related comments.
Rephrase text to avoid links in regress.sgml

Otherwise, the standalone regress_README build gets unhappy.
Fix comparison of an array of characters with zero to compare with '\0' instead.

Report from Andres Freund.
Improve connection-failure error handling in contrib/postgres_fdw.

postgres_fdw tended to say "unknown error" if it tried to execute a command
on an already-dead connection, because some paths in libpq just return a
null PGresult for such cases.  Out-of-memory might result in that, too.
To fix, pass the PGconn to pgfdw_report_error, and look at its
PQerrorMessage() string if we can't get anything out of the PGresult.

Also, fix the transaction-exit logic to reliably drop a dead connection.
It was attempting to do that already, but it assumed that only connection
cache entries with xact_depth > 0 needed to be examined.  The folly in that
is that if we fail while issuing START TRANSACTION, we'll not have bumped
xact_depth.  (At least for the case I was testing, this fix masks the
other problem; but it still seems like a good idea to have the PGconn
fallback logic.)

Per investigation of bug #9087 from Craig Lucas.  Backpatch to 9.3 where
this code was introduced.
PL/Perl: Fix compiler warning

The code was assigning a (Datum) 0 to a void pointer.  That creates a
warning from clang 3.4.  It was probably a thinko to begin with.
Minor improvements to replication slot documentation.

Fix a thinko pointed out by Jeff Davis, and convert a couple of other
references into links.
Remove unnecessary relcache flushes after changing btree metapages.

These flushes were added in my commit d2896a9ed, which added the btree
logic that keeps a cached copy of the index metapage data in index relcache
entries.  The idea was to ensure that other backends would promptly update
their cached copies after a change.  However, this is not really necessary,
since _bt_getroot() has adequate defenses against believing a stale root
page link, and _bt_getrootheight() doesn't have to be 100% right.
Moreover, if it were necessary, a relcache flush would be an unreliable way
to do it, since the sinval mechanism believes that relcache flush requests
represent transactional updates, and therefore discards them on transaction
rollback.  Therefore, we might as well drop these flush requests and save
the time to rebuild the whole relcache entry after a metapage change.

If we ever try to support in-place truncation of btree indexes, it might
be necessary to revisit this issue so that _bt_getroot() can't get caught
by trying to follow a metapage link to a page that no longer exists.
A possible solution to that is to make use of an smgr, rather than
relcache, inval request to force other backends to discard their cached
metapages.  But for the moment this is not worth pursuing.