METALOG, unless manually overwritten, is defined as ${DESTDIR}/${DISTDIR}/METALOG

METALOG, unless manually overwritten, is defined as ${DESTDIR}/${DISTDIR}/METALOG

In the create-world-packages target we manually piece this together (unless
it is undefined), without the DISTDIR.  Normally DISTDIR is empty (unset) and
no one notices.  Now DISTDIR is a well known long-standing PORTS environment
variable and if that is set in the local environment the path to METALOG
is wrong as it no longer is ${DESTDIR}/METALOG.

Long-term we should start to avoid "publicly well known" names for global
variables, for now just piece ${DISTDIR} in as well.  This allows
create-world-packages to continue if DISTDIR is set in the env.