bsdinstall release: Start installer on multiple consoles

Currently the installer is only started on the primary ("high level")
console. For systems where this is the video console and serial consoles
aren't of interest, and headless systems with just a serial console,
this works just fine, but for systems where both video and serial
consoles are present and meaningful this requires the user to select the
right primary console in loader, with the poor user experience of the
system appearing to hang if they leave the wrong one selected. This
notably differs from our multi-user behaviour of spawning getty on every
console, where the only issue with selecting the wrong primary console
is a quieter boot process until the login prompt appears (or the system
crashes).

Instead, use the newly-added runconsoles helper to run the installer on
every console (except for ttyv*, where only ttyv0 will be used). For
interactive installations, any of the consoles can be used, though only
one should be used at a time as no effort is made to avoid multiple
installations running at the same time clobbering each other. If the
Live CD option is selected, the other installers (which should, if the
user is well-behaved, be sitting at the welcome screen) will be killed.
If an automated install is in use, the primary console will be used to
display its output, and the others will direct the user to the primary
console.

Reviewed by: brooks, gjb
Differential Revision: https://reviews.freebsd.org/D36805