tzset_basic: only call issetugid(2) if TZ has changed

tzset_basic: only call issetugid(2) if TZ has changed
If we are just going to return without parsing TZ, there is no need
to call issetugid(2) first.  We only need to call issetugid(2) the
first time TZ is checked or when the value of TZ has changed.
Previously, we called issetugid(2) for every call to the functions
described by localtime(3).  OK deraadt@