Draft: Apply glib backports to xdgmime

Backporting some commits from glib to xdgmime (see MR 2029: Updating xdgmime).

This patch set include the following glib commits:

• a1bfe899: Don't give up too early when collecting mime types
• e7822bd3: Silence an uninitialize variable warning
• e4d18133: xdgmime: Finer handling for cases where mmap() is not available
• b765c795: gio/xdgmime/: LGPLv2+ -> LGPLv2.1+
• 38869ece: xdgmime: Prevent infinite loops from badly-formed MIME registrations

These two commits have been skipped for now (some problems at merge time):

• 14e46ca2: Don't compile some unused functions in gio/xdgmime/
• fa6f4553: xdgmime: Add xdg_mime_set_dirs() method to override XDG envvars