Remove dependency on files in /usr/bin

In order to reduce the pre-requisites of this file, implement the
pattern matching and creation of a temporary test directory without
use of grep respectively mktemp.

The new version makes it possible to provide a writable /tmp in any
case and independently of other local or remote file systems (except /
and /dev) being mounted.

The use of "dd if=/dev/random" has the same dependency on /dev/random
being operational as the previous version that used "mktemp". If this
is found to be an issue on platforms that do not have gathered
sufficient entropy at the time when this scriot is run, I suggest to
replace the "dd" command with "ps lauxww" to get a somewhat random
test directory name.

Approved by:	rgrimes, glebius, cy
MFC after:	4 weeks
Differential Revision:	https://reviews.freebsd.org/D28209

libexec/rc/rc.d/tmp

@@ -40,10 +40,16 @@ load_rc_config $name
 
 mount_tmpmfs()
 {
-	if ! /bin/df /tmp | grep -q "^/dev/md[0-9].* /tmp"; then
-		mount_md ${tmpsize} /tmp "${tmpmfs_flags}"
-		chmod 01777 /tmp
-	fi
+	while read line; do
+		case $line in
+		/dev/md[0-9]*\ /tmp)
+			return;;
+		esac
+	done <<*EOF
+$(df /tmp)
+*EOF
+	mount_md ${tmpsize} /tmp "${tmpmfs_flags}"
+	chmod 01777 /tmp
 }
 
 # If we do not have a writable /tmp, create a memory
@@ -52,7 +58,8 @@ mount_tmpmfs()
 #
 case "${tmpmfs}" in
 [Aa][Uu][Tt][Oo])
-	if _tmpdir=$(mktemp -d -q /tmp/.diskless.XXXXXX); then
+	_tmpdir=/tmp/.diskless.$(dd if=/dev/random bs=32 count=1 status=none | sha256)
+	if mkdir -m 0700 ${_tmpdir}; then
 		rmdir ${_tmpdir}
 	else
 		if [ -h /tmp ]; then