bdfload: Teach it to write wsf font files.

wsfontload(8) now groks simple "wsf" file format - that is basically a
header of kinda struct wsdisplay_font but with string fields embedded
and numeric fields in little-endian and font data afterwards.

Teach bdfload to write out such files if -o file.wsf argument is
specified.

While here fix a couple of bugs too (sorry, should have been separate
commits).  Fix an off by one in numchars calculations.  Fix the type
of buflen.

(uwe)