The IO library was in io/ because I was too lazy to find autotools'
SUBDIRS rules at the time and that you could use '.' for the current
directory. Fix that and use its own error defines instead of
the system settings service. Clean up a for more things for good
measure too (like KEYFILE_DIR, etc).
Clean up handling of "special" keys in keyfiles, ie ones that
need more processing than the basic GKeyFile API supports. Add
MAC address reading (writing support to come).
Additionally, add some test bits for the keyfile plugin that get
run on 'make check'.
