bsdinstall: Add a new runconsoles helper binary

This helper binary will run a given command on every on console, as
defined by /etc/ttys (except for ttyv*, where only ttyv0 will be used).
If one of the command processes exits, the rest will be killed. This
will be used by a future change to start the installer on multiple
consoles.

Reviewed by: brooks, imp, gjb
Differential Revision: https://reviews.freebsd.org/D36804