XDG directory spec

Fix for #109

This is a full implementation that uses $XDG_CONFIG_HOME and $XDG_CONFIG_DIRS.
The rationale is that since CONFIG_HOME corresponds to /etc and DATA_HOME corresponds to /usr/share, everything should go in CONFIG_HOME.

This currently breaks support on Windows and Mac, so I'd like some pointers on what the config directories on those platforms are.
strdup can fail. Are all callers ok with the function effectively returning NULL? The sprintf (called by MP_VERBOSE) isn't.