tmpfiles: Recognize type 'v' (create btrfs subvol)

This change does NOT implement btrfs subvol creation.  Instead, it
treats 'v' the same as 'd', which is an acceptable fallback
according to the manual.

Fixes #58

sh/tmpfiles.sh.in

@@ -141,6 +141,13 @@ _D() {
 	fi
 }
 
+_v() {
+	# Create a subvolume if the path does not exist yet and the file system
+	# supports this (btrfs). Otherwise create a normal directory.
+	# TODO: Implement btrfs subvol creation.
+	_d "$@"
+}
+
 _L() {
 	# Create a symlink if it doesn't exist yet
 	local path=$1 mode=$2 uid=$3 gid=$4 age=$5 arg=$6
@@ -336,7 +343,7 @@ for FILE in $tmpfiles_d ; do
 
 		# whine about invalid entries
 		case $cmd in
-			f|F|w|d|D|p|L|c|C|b|x|X|r|R|z|Z) ;;
+			f|F|w|d|D|v|p|L|c|C|b|x|X|r|R|z|Z) ;;
 			*) warninvalid ; continue ;;
 		esac
 
@@ -344,7 +351,7 @@ for FILE in $tmpfiles_d ; do
 		if [ "$mode" = '-' -o "$mode" = '' ]; then
 			case "$cmd" in
 				p|f|F) mode=0644 ;;
-				d|D) mode=0755 ;;
+				d|D|v) mode=0755 ;;
 				C|z|Z|x|r|R|L) ;;
 			esac
 		fi