Add --with-dracutdir configure option

The standard dracut directory has moved from /usr/share/dracut to
/usr/lib/dracut. To ensure the dracut modules get installed in
the correct location provide a --with-dracutdir configure option
to set the path.

The default install location has been updated to /usr/lib/dracut
which is used by more current versions of Fedora. However, this
default is overriden by the RPM packaging for consistency.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>