Fix up some problems in handling of zic-style time zone names in datetime

Fix up some problems in handling of zic-style time zone names in datetime
input routines.  Remove the former "DecodePosixTimezone" function in favor of
letting the zic code handle POSIX-style zone specs (see tzparse()).  In
particular this means that "PST+3" now means the same as "-03", whereas it
used to mean "-11" --- the zone abbreviation is effectively just a noise word
in this syntax.  Make sure that all named and POSIX-style zone names will be
parsed as a single token.  Fix long-standing bogosities in printing and input
of fractional-hour timezone offsets (since the tzparse() code will accept
these, we'd better make 'em work).  Also correct an error in the original
coding of the zic-zone-name patch: in "timestamp without time zone" input,
zone names are supposed to be allowed but ignored, but the coding was such
that the zone changed the interpretation anyway.