add full path to font selector in image_captcha

This patch adds the full path to the font selector in image_captcha, so the site administrator knows better which font he selects.

This could also prevent a possible security issue. When there is an exploitable bug (e.g. buffer overflow) in the TTF-handling code of the PHP image library, a malicious user with upload permissions could upload a dangerous TTF file with the same name of an already available font. Without the full path, the site administrator has no means to determine which selection he has to make.