mtab: move toward requiring /etc/mtab to be a symbolic link

This changes the mtab service in the following way:

- If /etc/mtab is a symbolic link, success is returned.
- If /etc is not writable, we warn that we could not update /etc/mtab
  and return success.
- If /etc/mtab does not exist, we create a symbolic link from
  /etc/mtab to /proc/self/mounts.
- Otherwise, we warn that updating /etc/mtab as a file is
  deprecated and continue to update it after outputting instructions to
  the user for how to move it to a symbolic link.

init.d/mtab.in

@@ -12,28 +12,30 @@ depend()
 
 start()
 {
-	if [ -L /etc/mtab ]; then
-		return 0
-	fi
-
+	[ -L /etc/mtab ] && return 0
+	local rc=0
 	ebegin "Updating /etc/mtab"
-	vewarn "The support for updating /etc/mtab as a file is"
-	vewarn "deprecated and will be removed in the future."
-	vewarn "Please run the following command as root on your system."
-	vewarn
-	vewarn "ln -snf /proc/self/mounts /etc/mtab"
-	if ! echo 2>/dev/null >/etc/mtab; then
-		ewend 1 "/etc/mtab is not updateable"
-		return 0
+	if ! checkpath -W /etc; then
+		rc=1
+	elif [ ! -e /etc/mtab ]; then
+		ln -snf /proc/self/mounts /etc/mtab
+	else
+		ewarn "The support for updating /etc/mtab as a file is"
+		ewarn "deprecated and will be removed in the future."
+		ewarn "Please run the following command as root on your system."
+		ewarn
+		ewarn "ln -snf /proc/self/mounts /etc/mtab"
+		ewarn
+
+		# With / as tmpfs we cannot umount -at tmpfs in localmount as that
+		# makes / readonly and dismounts all tmpfs even if in use which is
+		# not good. Luckily, umount uses /etc/mtab instead of /proc/mounts
+		# which allows this hack to work.
+		grep -v "^[! ]* / tmpfs " /proc/mounts > /etc/mtab
+
+		# Remove stale backups
+		rm -f /etc/mtab~ /etc/mtab~~
 	fi
-
-	# With / as tmpfs we cannot umount -at tmpfs in localmount as that
-	# makes / readonly and dismounts all tmpfs even if in use which is
-	# not good. Luckily, umount uses /etc/mtab instead of /proc/mounts
-	# which allows this hack to work.
-	grep -v "^[! ]* / tmpfs " /proc/mounts > /etc/mtab
-
-	# Remove stale backups
-	rm -f /etc/mtab~ /etc/mtab~~
-	eend 0
+	eend $rc "/etc is not writable; unable to create /etc/mtab"
+	return 0
 }