vt fontcvt: move to usr.bin/vtfontcvt

vtfontcvt is useful for end users to convert arbitrary bitmap fonts
for use by vt(4). It can also be used as a build tool, allowing us
to keep the source font data in the src tree rather than uuencoded
binaries.

Reviewed by: ray, wblock (D183)
Sponsored by: The FreeBSD Foundation