support RFC 1123 times with parse_time/3

[mndrix]

SWI Prolog already has the predicate parse_time/3 which handles RFC 1123.

It would be easy and convenient to support parsing for that format by using the built in. Going the other direction would be handled with format_time/3

[mndrix]

As of commit 4c55702 parsing is supported through a UI hack. Parse the RFC 1123 string with rfc3339(...) and everything works. This is ugly and doesn't support generating these timestamps.