Prevent using strncpy with src == dest in TupleDescInitEntry.

The C and POSIX standards state that strncpy's behavior is undefined when
source and destination areas overlap.  While it remains dubious whether any
implementations really misbehave when the pointers are exactly equal, some
platforms are now starting to force the issue by complaining when an
undefined call occurs.  (In particular OS X 10.9 has been seen to dump core
here, though the exact set of circumstances needed to trigger that remain
elusive.  Similar behavior can be expected to be optional on Linux and
other platforms in the near future.)  So tweak the code to explicitly do
nothing when nothing need be done.

Back-patch to all active branches.  In HEAD, this also lets us get rid of
an exception in valgrind.supp.

Per discussion of a report from Matthias Schmitt.
Fix some odd behaviors when using a SQL-style simple GMT offset timezone.

Formerly, when using a SQL-spec timezone setting with a fixed GMT offset
(called a "brute force" timezone in the code), the session_timezone
variable was not updated to match the nominal timezone; rather, all code
was expected to ignore session_timezone if HasCTZSet was true.  This is
of course obviously fragile, though a search of the code finds only
timeofday() failing to honor the rule.  A bigger problem was that
DetermineTimeZoneOffset() supposed that if its pg_tz parameter was
pointer-equal to session_timezone, then HasCTZSet should override the
parameter.  This would cause datetime input containing an explicit zone
name to be treated as referencing the brute-force zone instead, if the
zone name happened to match the session timezone that had prevailed
before installing the brute-force zone setting (as reported in bug #8572).
The same malady could affect AT TIME ZONE operators.

To fix, set up session_timezone so that it matches the brute-force zone
specification, which we can do using the POSIX timezone definition syntax
"<abbrev>offset", and get rid of the bogus lookaside check in
DetermineTimeZoneOffset().  Aside from fixing the erroneous behavior in
datetime parsing and AT TIME ZONE, this will cause the timeofday() function
to print its result in the user-requested time zone rather than some
previously-set zone.  It might also affect results in third-party
extensions, if there are any that make use of session_timezone without
considering HasCTZSet, but in all cases the new behavior should be saner
than before.

Back-patch to all supported branches.
Ensure all files created for a single BufFile have the same resource owner.

Callers expect that they only have to set the right resource owner when
creating a BufFile, not during subsequent operations on it.  While we could
insist this be fixed at the caller level, it seems more sensible for the
BufFile to take care of it.  Without this, some temp files belonging to
a BufFile can go away too soon, eg at the end of a subtransaction,
leading to errors or crashes.

Reported and fixed by Andres Freund.  Back-patch to all active branches.
Changed test case slightly so it doesn't have an unused typedef.