libkmod-config,depmod: Accept special files as configuration files, too

If we can open it and read it, it's good enough for us. Otherwise, we
cannot use -C /dev/null to skip the system configuration for instance:

$ ./tools/modprobe -C /dev/null -c
libkmod: ERROR libkmod/libkmod-config.c:821 conf_files_list: unsupported
file mode /dev/null: 0x21b6
...