utf8_encode now returns the number of characters copied
simplified code accordingly
made local functions static
fixed shadowing issues
added __attr_printf() attributes for gcc to check printf arguments
use snprintf() instead of sprintf()
- select_font() now locks the font, which must be unlocked with
release_font(). Extra locks can be added with lock_font().
- Added release_font() wherever needed.
- Added put_error() to store errors in *errors* system buffer.
- Make put_status() store messages in *messages* system buffer.
