Allow overriding default builtin-gconf values via config files

MCE reads files pattern "/etc/mce/[0-9][0-9]<feature>.conf" and uses
the content to modify defaults of builtin-gconf values.

The processing is done in alphabetical order. If several files modify
the same key, the value from the latest one will be used.

The /var/lib/mce/builtin-gconf.values will hold delta to the situation
after processing the config files in /etc/mce directory.

[mce] Allow changing config defaults via installing files to /etc/mce