Move font related data structured to sys/font.c and update vtfontcvt

Prepare support to be able to handle font data in loader, consolidate
data structures to sys/font.h and update vtfontcvt.

vtfontcvt update is about to output set of glyphs in form of C source,
the implementation does allow to output compressed or uncompressed font
bitmaps.

Reviewed by: bcr
Differential Revision: https://reviews.freebsd.org/D24189