pgsql: Avoid unportable strftime() behavior in pg_dump/pg_dumpall.

Avoid unportable strftime() behavior in pg_dump/pg_dumpall.

Commit ad5d46a4494b0b480a3af246bb4227d9bdadca37 thought that we could
get around the known portability issues of strftime's %Z specifier by
using %z instead. However, that idea seems to have been innocent of
any actual research, as it certainly missed the facts that
(1) %z is not portable to pre-C99 systems, and
(2) %z doesn't actually act differently from %Z on Windows anyway.

Per failures on buildfarm member hamerkop.

While at it, centralize the code defining what strftime format we
want to use in pg_dump; three copies of that string seems a bit much.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/f455fcfdb8ca3b67373223a4e15648c35e2592a9

Modified Files
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src/bin/pg_dump/dumputils.h | 17 +++++++++++++++++
src/bin/pg_dump/pg_backup_archiver.c | 8 +++++---
src/bin/pg_dump/pg_dumpall.c | 6 +++---
3 files changed, 25 insertions(+), 6 deletions(-)