gdatetime: Extend the '%z' timezone format

Implement gnulib strftime extensions for the '%z' numeric timezone
format. These are also supported and documented by GNU date(1):

%z     +hhmm numeric time zone (e.g., -0400)

%:z    +hh:mm numeric time zone (e.g., -04:00)

%::z   +hh:mm:ss numeric time zone (e.g., -04:00:00)

%:::z  numeric time zone with : to necessary precision (e.g., -04, +05:30)

https://bugzilla.gnome.org/show_bug.cgi?id=707151