dracut-functions.sh: support for alternative udev dirs - udevaltdirs

It is required for Gentoo which moves udev from / to /usr and supports
both /lib/udev and /usr/lib/udev for compatibility with other packages.

Credits go to Alexander Tsoy <alexander@tsoy.me>.
gentoo.conf: let udevdir= be handled by pkg-config and use udevaltdirs

To avoid need of rebuild after udev update/downgrade let udevdir= be set
dynamically with pkg-config.  Use udevaltdirs to search for rules both
in old and new locations.