keyfile: clean up directory structure

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).

system-settings/plugins/keyfile/nm-keyfile-connection.c

@@ -31,6 +31,7 @@
 #include "nm-keyfile-connection.h"
 #include "reader.h"
 #include "writer.h"
+#include "common.h"
 
 static NMSettingsConnectionInterface *parent_settings_connection_iface;