MFC r262957, r267591, r289634:

r262957 (marcel):
Change the terminal type/class for enabled serial lines to 3wire. This
allows us to change the uart(4) driver to not hardcode specific line
settings for the serial console.

A terminal type of 3wire makes sure the console still works when no DCD
signal is present, which preserves behviour. When it is known that the
terminal server (or DCE in general) provides DCD, a terminal type/class
of std can be used. This has the effect of being logged out when one
disconnects from the console -- improving security overall.

r267591 (grehan):
Convert the potential console port over to using 3wire, for i386/amd64.

r289634:
Enable all callin ttys if the tty is an available console.

Sponsored by: The FreeBSD Foundation

MFC r260913,266895:
Add a new flag to /etc/ttys: onifconsole. This is equivalent to "on" if the
device is an active kernel console and "off" otherwise. This is designed to
allow serial-booting x86 systems to provide a login prompt on the serial line
by default without providing one on all systems by default. Set this flag
on x86 systems for ttyu0.

Comments and suggestions by: grehan, dteske, jilles

MFC r260913,266895:
Add a new flag to /etc/ttys: onifconsole. This is equivalent to "on" if the
device is an active kernel console and "off" otherwise. This is designed to
allow serial-booting x86 systems to provide a login prompt on the serial line
by default without providing one on all systems by default. Set this flag
on x86 systems for ttyu0.

Comments and suggestions by: grehan, dteske, jilles