MFC r260913,266895:

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

git-svn-id: svn://svn.freebsd.org/base/stable/10@267236 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f