Allow console drivers active from early boot to be used with xf86-video-scfb,
rather than only drivers attached later on. This involves a small amount of
code duplication with dev/fb/fbd.c, which will fixed later on.

Also improve performance of vt_blank() by making it not read from the
framebuffer unnecessarily.
Allow efifb to be used with xf86-video-scfb. This is important for EFI
systems without either a CSM or real graphics drivers, such as my Lenovo
Haswell laptop.

This provides working X with the small complication of a console cursor
permanently overlaid on the upper-left corner of the screen that will be
dealt with later.

Also remove some redundant screen clearing.
Revert previous revision (r268461) for reasons documented in PR.
To use tmux in an rc.d script, use the new-session flag "-d".
To use screen in an rc.d script, use the "-dm" flag.
If you really need to launch an attached session, manually
export TERM=xterm (FreeBSD 9.0 or higher) or export TERM=cons25
for older releases.

Reported by:	bdrewery
Discussed on:	src-committers, svn-src-all, svn-src-head
PR:		bin/191869