Fix setting of the border color. Teken doesn't support syscons' escape
sequence "ESC [ %d A" for this although that was used here. I will fix
teken later, but use the more portable ioctl KDSBORDER here.

The ioctl is also much easier to use if you check that it works. For
-b, check it and complain and exit if it failed, so that it is more
obvious that that vt doesn't support border colors. Don't check it
when restoring the border color in revert(), since revert() is used
on vt for handling other errors.

Fix nearby error handling and style. For the error of an invalid
color, revert() and print a specific error message using err() instead
of not revert()ing and printing spam using usage().