gdatetime: Fix locale handling for nl_langinfo() calls

With the various macros we use to provide fallbacks for missing
nl_langinfo() fields, the locale handling can become quite complex:
nl_langinfo() returns strings encoded in the current locale, but C_()
returns strings encoded in UTF-8 (by GLib convention — you do actually
need to call bind_textdomain_codeset() to achieve this).

There are various format specifiers, especially with the new %Ob, %OB,
%Oh specifiers, which conditionally call nl_langinfo() or something
based on C_(). This makes encoding handling difficult.

Add additional macros which indicate whether the macros they’re paired
with return something encoded in the current locale, or encoded in
UTF-8. The user of the macro can then use these to work out whether to
re-encode.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=793578