Refactor to centralize the setting of tzname.

See Christos Zoulas in:
http://mm.icann.org/pipermail/tz/2014-November/021881.html
* localtime.c (update_tzname_etc): New function.
(settzname, localsub): Use it.
(localsub): Rename parameter for clarity.
tzalloc should scrub abbreviations, compatibly with tzset.

* localtime.c (settzname): Move scrubbing into ...
(scrub_abbrs): ... this new function.
(zoneinit): Use it here, rather than in settzname.
* NEWS: Document this.