First step toward making the xfsdump inventory not break the FHS spec.

First step toward making the xfsdump inventory not break the FHS spec.
Previously, xfsdump wrote to /var/xfsdump for all its inventory data
needs.  This is now a runtime decision - for existing installations we
continue doing just that, for new installations we use the compliant-
with-FHS directory /var/lib/xfsdump.  Note: cannot just mv one to the
other as xfsdump is "clever" enough to store full pathnames in all its
inventory file - this issue will be resolved by Ivan in an upcoming
xfsinvutil checkin.