MFC r316422:

MFC r316422:

Remove checks that background (bg) colors are not bright and buggy
attempts to keep them that way.  The bg brightness bit is interpreted
as blinking in some modes, but it would barely be useful to disallow
setting it when it would give blinking in code which knew when that
is.  The old code mostly knew this wrong, and added handling errors.
It is in fact impossible to know, since future mode switches may
change the meaning of the bit many times on the screen and in history.

Old versions of vidcontrol disallowed bg color numbers >= 8 in all
cases.  This is very VGA/syscons-centric.  Syscons uses the VGA defaults
of blinking fg instead of bright bg in text mode and bright bg in
graphics mode.  On VGA, this is very easy to toggle at any time, and
vt blows away the VGA text mode default at boot time.

r146736 changed this to try to allow bg color numbers in graphics mode
only.  This is even more VGA/syscons-centric, and there are many bugs
in this, and many nearby bugs in the parser.  These are increased or
decreased by differences and bugs in vt and teken.

Perhaps the most obvious bug was that almost any vidcontrol command
which changes any color or the mode causes an error if the initial fg
color is bright.  E.g., in syscons text mode, after "vidcontrol
lightwhite" to make the fg bright, another "vidcontrol lightwhite" is
rejected and buggy fixup code changes the fg to white.  This is because
the bright fg color creates a bright bg color for the phantom reverse
video attribute, so was rejected.  (The reverse video attribute is
phantom because teken ignores the user's setting of it and simply
reverses the fg attributes to create the bg attributes.  Sometimes
some layer masks off the brightness/blinking bit, but not here.)

Perhaps the next most obvious one was that "vidcontrol lightgreen
lightblue" was misparsed as 2 settings of the fg instead of 1 setting
of the fg and 1 invalid setting of the bg.  This is because the
parser supports an undocumented syntax with many parsing bugs (an
ambiguity gives this one).

I recently fix bugs in teken that broke setting of bright fg's and
bg's in the normal way.  This gave more settings of then, so the old
bugs showed up more often.