Support setting the colors of cursors for the VGA renderer.

Advertise this by changing the defaults to mostly red. If you don't like
this, change them (almost) back using:

vidcontrol -c charcolors,base=7,height=0
vidcontrol -c mousecolors,base=0[,height=15]

The (graphics mode only) mouse cursor colors were hard-coded to a black
border and lightwhite interior. Black for the border is the worst
possible default, since it is the same as the default black background
and not good for any dark background. Reversing this gives the better
default of X Windows. Coloring everything works better still. Now
the coloring defaults to a lightwhite border and red interior.

Coloring for the character cursor is more complicated and mode
dependent. The new coloring doesn't apply for hardware cursors. For
non-block cursors, it only applies in graphics mode. In text mode,
the cursor color was usually a hard-coded (dull)white for the background
only, unless the foreground was white when it was a hard-coded black
for the background only, unless the foreground was white and the
background was black it was reverse video. In graphics mode, it was
always reverse video for the block cursor. Reverse video is worse,
especially over cutmarking regions, since cutmarking still uses simple
reverse video (nothing better is possible in text mode) and double
reverse video for the cursor gives normal video. Now, graphics mode
uses the same algorithm as the best case for text mode in all cases
for graphics mode. The hard-coded sequence { white, black, } for the
background is now { red, white, blue, } where the first 2 colors can
be configured. The blue color at the end is a sentinel which prevents
reverse video being used in most cases but breaks the compatibility
setting for white on black and black on white characters. This will
be fixed later. The compatibility setting is most needed for mono modes.

The previous commit to syscons.c changed sc_cnterm() to be more careful.
It followed null pointers in some cases. But sc_cnterm() has been
unreachable for 15+ years since changes for multiple consoles turned
off calls to the the cnterm destructor for all console drivers. Before
them, it was only called at boot time. So no driver with an attached
console has ever been unloadable and not even the non-console destructors
have been tested much.