Change NO_MAKEDEV to a finer granularity method:

NO_MAKEDEV_INSTALL and NO_MAKEDEV_RUN.  The former implying the latter.
The names imply what they do.  The last commit by DES based on a PR defeated
the original idea behind NO_MAKEDEV, which was not to run MAKEDEV, but to do
the installation of MAKEDEV.  This should satisfy both parties on the MAKEDEV
challenge.
Reflect this in mergemaster, this might later on be decided to be set to
NO_MAKEDEV_INSTALL, for now I kept to the old behaviour.

usr.sbin/mergemaster/mergemaster.sh

@@ -446,7 +446,7 @@ case "${RERUN}" in
       ;;
     esac
     make DESTDIR=${TEMPROOT} distrib-dirs &&
-    make DESTDIR=${TEMPROOT} -DNO_MAKEDEV distribution;} ||
+    make DESTDIR=${TEMPROOT} -DNO_MAKEDEV_RUN distribution;} ||
   { echo '';
     echo "  *** FATAL ERROR: Cannot 'cd' to ${SOURCEDIR} and install files to the";
     echo "      temproot environment";