Add init_exec kenv(1) variable, to make init(8) execute a file
after opening the console, replacing init as PID 1.

From the user point of view, it makes it possible to run eg the
shell as PID 1, using 'set init_exec=/bin/sh' at the loader(8)
prompt.

Reviewed by: kib
MFC after: 2 weeks
Relnotes: yes
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D16625