MFC 264242,264244,264259

Fix panic on load new driver while vt(4) is in VGA textmode.
o Mute terminal while vt(4) driver change in progress.
o Reset VDF_TEXTMODE before init new driver.
o Assign default font, if new driver is not in TEXTMODE.
o Do not update screen while driver changing.
o Unmute terminal when done with driver replacement.
o Move init fonts to early point.
o Minor cleanup.
o Do not fill screen, while muted. (kern/subr_terminal.c)

Sponsored by:	The FreeBSD Foundation