Allow timezone info to be specified in a file... /etc/TZ. Turned on by

Allow timezone info to be specified in a file... /etc/TZ.  Turned on by
default for now.  From the comments...

* Defining __TIME_TZ_FILE causes tzset() to attempt to read the TZ value
* from the file /etc/TZ if the TZ env variable isn't set.  The file contents
* must be the intended value of TZ, followed by a newline.  No other chars,
* spacing, etc is allowed.  As an example, an easy way for me to init
* /etc/TZ appropriately would be:    echo CST6CDT > /etc/TZ

Also optimized timezone setting when the timezone string hasn't changed,
as well as fixed a minor buglet wrt SUSv3-allowed chars in TZ std and
dst fields.