Running getty(8) on /dev/console when using a glass console interferes with

Running getty(8) on /dev/console when using a glass console interferes with
running Xorg in a way that isn't fully understood.  So change the arm64
install.md to munge /etc/ttys to enable the ttyC0 entry if we detect that
wsdisplay0 is the console and make sure the code in install.sub that
does the actual munging disables the console entry before enabling another
entry to prevent running two getty(8) processes on (effectively) the same
device.

ok deraadt@