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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REL8_3_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/4d3482a741a5f6c7594e60430cadb0f15024cab2

Modified Files
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src/backend/utils/adt/datetime.c |    6 ++----
src/include/utils/timestamp.h    |   10 ++++++++++
2 files changed, 12 insertions(+), 4 deletions(-)