Fix assorted breakage in to_char()'s OF format option.
In HEAD, fix incorrect field width for hours part of OF when tm_gmtoff is
negative. This was introduced by commit
2d87eedc1d4468d3 as a result of
falsely applying a pattern that's correct when + signs are omitted, which
is not the case for OF.
In 9.4, fix missing abs() call that allowed a sign to be attached to the
minutes part of OF. This was fixed in 9.5 by
9b43d73b3f9bef27, but for
inscrutable reasons not back-patched.
In all three versions, ensure that the sign of tm_gmtoff is correctly
reported even when the GMT offset is less than 1 hour.
Add regression tests, which evidently we desperately need here.
Thomas Munro and Tom Lane, per report from David Fetter