libkmod-config: refactor functions to get config

libkmod-config: refactor functions to get config

It makes more sense to have libkmod-config.c deal with the configuration
directly and the others get the config from ctx. As a bonus point we get
a smaller binary. Following numbers are for x86-64, libkmod + kmod:

Before:
   text	   data	    bss	    dec	    hex	filename
 128840	   1496	    104	 130440	  1fd88	tools/modprobe

After:
   text	   data	    bss	    dec	    hex	filename
 128392	   1496	    104	 129992	  1fbc8	tools/modprobe