lib/getdate.y: Ignore time-zone information and use UTC

lib/getdate.y: Ignore time-zone information and use UTC

There is exactly one caller of this function, and it wants a date, not a
time.  It is useless to be able to parse local dates, because we
ultimately store a UTC date.  To avoid confusion, unconditionally use
UTC.  Since this code had important bugs regarding offset, we can safely
assume that no existing users rely on being able to use their local
date (this never worked correctly).

Also, the code parsing time zones is quite bad, for today's standards.

Link: <https://github.com/shadow-maint/shadow/issues/1202>
Link: <https://github.com/shadow-maint/shadow/issues/1209>
Reported-by: Chris Hofstaedtler <zeha@debian.org>
Reported-by: Tim Parenti <tim@timtimeonline.com>
Reported-by: Lee Garrett <lgarrett@rocketjump.eu>
Cc: Gus Kenion <https://github.com/kenion>
Cc: Michael Vetter <jubalh@iodoru.org>
Cc: Paul Eggert <eggert@cs.ucla.edu>
Cc: Iker Pedrosa <ipedrosa@redhat.com>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Cc: Brian Inglis <Brian.Inglis@SystematicSW.ab.ca>
Signed-off-by: Alejandro Colomar <alx@kernel.org>