Enable vt(4) by default

vt(4) is a new console driver which brings features such as:

o  Support for Unicode and double-width characters
o  Integration with the KMS kernel video drivers
o  Support for UEFI

You may need to update your console settings in /etc/rc.conf, most
probably the keymap. During boot, /etc/rc.d/syscons will indicate what
you need to do.

vt(4) still has issues and lacks some features compared to syscons(4).
See the wiki for up-to-date information:

https://wiki.freebsd.org/Newcons

If you want to keep using syscons(4), you can do so by adding the
following line to /boot/loader.conf:

kern.vty=sc

Differential Revision: https://reviews.freebsd.org/D1005
Discussed with: emaste@, nwhitehorn@, ray@
Relnotes: yes