pgsql: Expand the allowed range of timezone offsets to +/-15:59:59 from

Expand the allowed range of timezone offsets to +/-15:59:59 from Greenwich.

We used to only allow offsets less than +/-13 hours, then it was +/14,
then it was +/-15. That's still not good enough though, as per today's bug
report from Patric Bechtel. This time I actually looked through the Olson
timezone database to find the largest offsets used anywhere. The winners
are Asia/Manila, at -15:56:00 until 1844, and America/Metlakatla, at
+15:13:42 until 1867. So we'd better allow offsets less than +/-16 hours.

Given the history, we are way overdue to have some greppable #define
symbols controlling this, so make some ... and also remove an obsolete
comment that didn't get fixed the last time.

Back-patch to all supported branches.

Branch
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REL9_0_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/9b0875a2045cecc9de2a0f1c16e7930510a394ae

Modified Files
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src/backend/utils/adt/date.c | 5 ++---
src/backend/utils/adt/datetime.c | 6 ++----
src/include/utils/timestamp.h | 10 ++++++++++
3 files changed, 14 insertions(+), 7 deletions(-)