Use sorted configuration files in precedence order

Use sorted configuration files in precedence order

We need to keep config files sorted and use them taking the precedence
order into account.

The following message was taken from module-init-tools commit doing a
similar thing:

	Configuration files are parsed in alphabetic order, regardles of
	what directory they reside in. Furthermore, if several files by
	the same name exist in different directories only the one in the
	directory with highest precedence is loaded.

	The order of precedence is /run, /etc, /usr/lib, /lib.

The sad thing is that we are not using openat() anymore since each file
is in different directories. In future we might change the
implementation to open all DIRs and keep a reference
to them instead of the path. However we'd have to keep a separate list
with all the opened dirs so we can close them later (when all configs
are parsed).