Enable a login console on ttyu0 by default if it is a serial console. This

Enable a login console on ttyu0 by default if it is a serial console. This
prevents surprising effects post-installation on serial-only x86 systems
(e.g. bhyve) where the kernel messages will appear, rc scripts will run,
and then there will be no login prompt and you will be locked out of the
system.

Thanks to Devin Teske and Peter Grehan for ideas and testing. With this
patch, it may also be possible to unify all or most of the machine-specific
ttys files.