If the /sbin/init binary is broken somehow, the way out is to set the
loader environment variable init_path to something else. The most
natural choice would be either /bin/sh or /rescue/sh. Unfortunately,
this does not work because the init process starts withoud stdin/out
descriptors.
Make it nicer to users by teaching /bin/sh startup code to open standard
descriptors on /dev/console if the shell is run as init.
Reviewed by: imp, jilles, zlei
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D56536
Konstantin Belousov
d7338bb491