jilles@ pointed out that using ${PRUNEDIRS:=".zfs"} in updatedb.sh

made it impossible to override PRUNEDIRS to make it empty.  Use the
non-colon form to only set PRUNEDIRS if it is completely unset.  (For
parallelism, the other configuration defaults here could be done the
same way, but that could be more obviously accomplished by disabling
updatedb in periodic.conf, so leave them alone for now.)

usr.bin/locate/locate/updatedb.sh

@@ -52,7 +52,7 @@ PATH=$LIBEXECDIR:/bin:/usr/bin:$PATH; export PATH
 : ${FCODES:=/var/db/locate.database}	 # the database
 : ${SEARCHPATHS:="/"}		# directories to be put in the database
 : ${PRUNEPATHS:="/tmp /usr/tmp /var/tmp /var/db/portsnap"} # unwanted directories
-: ${PRUNEDIRS:=".zfs"}	# unwanted directories, in any parent
+: ${PRUNEDIRS=".zfs"}	# unwanted directories, in any parent
 : ${FILESYSTEMS:="$(lsvfs | tail -n +3 | \
 	egrep -vw "loopback|network|synthetic|read-only|0" | \
 	cut -d " " -f1)"}		# allowed filesystems