Add load_os_font_list for linux in text-freetype2 plugin

This implements searching for available fonts on linux.
The code currently recurses through the directories /usr/share/fonts
and ~/.fonts and tries to open every file it finds with freetype.
Fix crash in text-freetype2 plugin

There seem to be some ancient font files out in the wild that don't
specify a font family, which may cause a crash when family_in is NULL.
This adds a NULL pointer check to add_font_path to avoid crashing.