[tz] 'date -u' should say "UTC", not "GMT"

[Paul Eggert]

Come to think of it, the tz implementation of 'date -u' should say "UTC" 
instead of "GMT", and in general the tz code and documentation should 
prefer UT or UTC to GMT whenever this would improve technical accuracy.  
Although "GMT" is the traditional time zone abbreviation output of 'date 
-u', POSIX has allowed "UTC" ever since IEEE Std 1003.1-1992.  
Outputting "UTC" is more technically correct, certainly for time stamps 
since 1961, and arguably even before that if one interprets "UTC" 
proleptically.  Also, outputting "UTC" is now a quite-common behavior, 
since it's the standard behavior in GNU/Linux.  So I'll look into 
proposing a patch to the tz code to have it support this behavior.  
There are probably a few other places in the code that should also 
prefer "UT" or "UTC" to "GMT".