vfs mock: Allow paths without leading /

We're not doing device names and FILO would only check for the leading
slash to distinguish the path from the device name. Otherwise, if a
device name is given, FILO would allow paths without leading slash. So
allow that, too, as it also eases test scripting.
diff --git a/src/vfs.c b/src/vfs.c
index 4a39f1b..943fea4 100644
--- a/src/vfs.c
+++ b/src/vfs.c
@@ -41,12 +41,6 @@
 
 int file_open(const char *filename)
 {
-	if (filename && filename[0] != '/') {
-		fprintf(stderr, "file_open: filename musst start with '/',"
-			"no support for device identifier");
-		return 0;
-	}
-	
 	if (!fsys) {
 		fprintf(stderr, "file_open: no open filesystem.\n");
 		return 0;