DED: Added special "no flags" value (0) for Flags

The correct syntax to specify that a Flags field has no value is:

Flags = ""; # Old format
Flags = 0;  # New format (added in this commit)

A value token must be included as with any other field, so the following
is illegal:

Flags = ;

doomsday/engine/portable/src/def_read.c

@@ -510,6 +510,17 @@ static int ReadFlags(int* dest, const char* prefix)
     *dest = 0;
 
     ReadToken();
+    if(ISTOKEN(";"))
+    {
+        SetError("Missing flags value.");
+        return false;
+    }
+    if(ISTOKEN("0"))
+    {
+        // No flags defined.
+        return true;
+    }
+
     UnreadToken(token);
     if(ISTOKEN("\""))
     {