Support old version of module-init-tools

Support old version of module-init-tools

modprobe included in version prior to 3.7 of module-init-tools doesn't
have -d | --dirname option which allows to give a prefix other than
'/' for kernel modules path. Dracut assumes existence of that
option and uses it even with default '/'. The patch passes -d option
only if it's different from default and also checks module-init-tools
version if user changes the prefix by --kmoddir Dracut option.