Correct some UTC-vs-UT solecisms.

In several places the code and documentation incorrectly used
"UTC" to describe time stamps that might precede the introduction
of UTC and for which UTC is therefore undefined.  Change these
uses to "UT", as that's the correct term when talking about these
time stamps.  Problem reported by Steve Allen in
<http://mm.icann.org/pipermail/tz/2013-September/019907.html>.
The major compatibility issue here is with 'zdump -v'; it'll now
output "UT" instead of the possibly-incorrect "UTC".
Many files change in minor ways in the commentary.
* zdump.c (show):
* zic.c (inzsub, addtype):
In output, say "UT" rather than "UTC", since the time stamp we're
talking about might precede the introduction of UTC.