I tried to make it absolutely correct but there are too many
conflicting definitions for the official list of time zones.
Since when we're parsing we know when to expect
a time zone and we know what they look like if not exactly
what the definitive set is, we compromise. We accept any
three-character sequence of upper case letters, possibly
followed by a capital T (all four-letter zones end in T).

There is one crazy special case (ChST) and the possibility
of a signed hour offset for GMT.

Fixes #3790
I hope forever, but I doubt that very much.

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/12969043