Fix for "playdemo" not working with custom home paths

This should fix https://mb.srb2.org/showthread.php?t=43205

EDIT: As a bonus I've gotten -playdemo and -timedemo to work again; however, the change involves may or may not be master-safe, so I'm merging this to next instead

src/d_netcmd.c

@@ -1452,7 +1452,12 @@ static void Command_Playdemo_f(void)
 
 	CONS_Printf(M_GetText("Playing back demo '%s'.\n"), name);
 
-	G_DoPlayDemo(name);
+	// Internal if no extension, external if one exists
+	// If external, convert the file name to a path in SRB2's home directory
+	if (FIL_CheckExtension(name))
+		G_DoPlayDemo(va("%s"PATHSEP"%s", srb2home, name))
+	else
+		G_DoPlayDemo(name);
 }
 
 static void Command_Timedemo_f(void)