timezone: Move to the XSI/POSIX definition for timezone.

The old timezone(3) function has long since been obsolete and has a
fatally flawed interface. Retain this function for compatibility
purposes, but shift to providing the offset from UTC in the timezone
variable, whether or not the timezone observes summer time in the
'daylight' variable. Document the tzname variable that's already been
set. Also make _tztab() static. It's not used in libc (or anywhere in
the tree) and it's not exported as a public dynamic symbol.

Sponsored by: Netflix
Reviewed by: brooks, kib
Differential Revision: https://reviews.freebsd.org/D44281