filesystem: implement .pk3dir convention

filesystem/filesystem.c

@@ -62,6 +62,7 @@ const fs_archive_t g_archives[] =
 {
 { "pak",    SEARCHPATH_PAK,    FS_AddPak_Fullpath, true },
 { "pk3",    SEARCHPATH_ZIP,    FS_AddZip_Fullpath, true },
+{ "pk3dir", SEARCHPATH_PK3DIR, FS_AddDir_Fullpath, true },
 { "wad",    SEARCHPATH_WAD,    FS_AddWad_Fullpath, false },
 { NULL }, // end marker
 };

filesystem/filesystem_internal.h

@@ -57,7 +57,8 @@ enum
 	SEARCHPATH_PLAIN = 0,
 	SEARCHPATH_PAK,
 	SEARCHPATH_WAD,
-	SEARCHPATH_ZIP
+	SEARCHPATH_ZIP,
+	SEARCHPATH_PK3DIR, // it's actually a plain directory but it must behave like a ZIP archive
 };
 
 typedef struct stringlist_s