gcredentials: add internal macros to simplify all the #ifdefs

Rather than having lots of obscure platform-based #ifdefs all over
gio, define some macros in gcredentialsprivate.h, and use those to
simplify the rest of the code.

https://bugzilla.gnome.org/show_bug.cgi?id=701482
gcredentials: add Hurd support

It uses the same system as FreeBSD.

https://bugzilla.gnome.org/show_bug.cgi?id=708265
gcredentials: add Solaris support

Based on patches from Igor Pashev.

https://bugzilla.gnome.org/show_bug.cgi?id=705029