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" ) )
 	{

code/qcommon/qcommon.h

@@ -1072,6 +1072,8 @@ void	* QDECL Sys_LoadGameDll( const char *name, intptr_t (QDECL **entryPoint)(in
 				  intptr_t (QDECL *systemcalls)(intptr_t, ...) );
 void	Sys_UnloadDll( void *dllHandle );
 
+qboolean Sys_DllExtension( const char *name );
+
 char	*Sys_GetCurrentUser( void );
 
 void	QDECL Sys_Error( const char *error, ...) __attribute__ ((noreturn, format (printf, 1, 2)));