[bdf] Use unsigned types. * src/bdf/bdf.h (bdf_glyph_t): Unsign `encoding'. (bdf_font_t): Unsign `default_char'. * src/bdf/bdfdrivr.h (BDF_encoding_el): Unsign `enc'. * src/bdf/bdflib.c (_bdf_add_property, _bdf_parse_glyphs, _bdf_parse_start): Updated accordingly. * src/bdf/bdfdrivr.c (bdf_cmap_char_{index,next}): Ditto.