Don't load libraries with non-standard file extensions

Also don't allow writting files ending in a library extension such
as ".so.0" or ".dylib.0".

code/qcommon/files.c

@@ -566,7 +566,7 @@ static void FS_CheckFilenameIsMutable( const char *filename,
 		const char *function )
 {
 	// Check if the filename ends with the library, QVM, or pk3 extension
-	if( COM_CompareExtension( filename, DLL_EXT )
+	if( Sys_DllExtension( filename )
 		|| COM_CompareExtension( filename, ".qvm" )
 		|| COM_CompareExtension( filename, ".pk3" ) )
 	{