accept only the expected UTC timezones in date parsing

HTTP/1.1 hardcodes GMT (RFC 7231 §7.1.1.1) and what is good enough for the
internet must be good enough for us™ as we reuse the implementation
internally to parse (most) dates we encounter in various places like the
Release files with their Date and Valid-Until header fields.

Implementing a fully timezone aware parser just feels too hard for no
effective benefit as it would take 5+ years (= until LTS's are out of
fashion) until a repository could use non-UTC dates and expect it to
work. Not counting non-apt implementations which might or might not
only want to encounter UTC here as well.

As a bonus, this eliminates the use of an instance of setlocale in
libapt.

Closes: 819697

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