Impelemnt ttys onifexists in init.

Implement a new init(8) option in /etc/ttys. If this option is present
on the entry in /etc/ttys, the entry will be active if and only if it
exists. If the name starts with a '/', it will be considered an
absolute path. If not, it will be a path relative to /dev.

This allows one to turn off video console getty that aren't present
(while running a getty on them even when they aren't the system
console). Likewise with serial ports.

It differs from onifconsole in only requiring the device exist rather
than it be listed as one of the system consoles.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D10037