Avoid calls to strlen in font processing functions.

Avoid calls to strlen in font processing functions.
* font.c (font_parse_name, font_parse_xlfd, font_parse_fcname)
(font_open_by_name): Changed to use length argument.  Adjust
users accordingly.
* font.h (font_open_by_name, font_parse_xlfd): Adjust prototypes.
* xfont.c (xfont_decode_coding_xlfd): Changed to return ptrdiff_t.
(xfont_list_pattern, xfont_match): Use length returned by
xfont_decode_coding_xlfd.
* xfns.c (x_default_font_parameter): Omit useless xstrdup.