sbin/init/init.c: Add init_exec

[trasz]

Summary

Add init_exec kenv(1) variable, to make init(8) execute the 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.

[trasz]

This also introduces execute_script() and refactors code a bit. My plan is to commit this part (refactoring) first, and the actual new functionality and man page updates later.

[kib]

Please split the review into refactor and new functionality. Lets handle the refactoring first.

[trasz]

Done, see https://reviews.freebsd.org/D16627.

[trasz]

Regen.

[kib]

sbin/init/init.c

340

Should be NULL.

Also, why do you use '_' namespace ?

And, I suggest to move the locals to the common declaration block.

[trasz]

sbin/init/init.c

340

The '_' is because I wan't to avoid shadowing argv - the one passed to main(). The declarations... Normally I'd do that, but decided to comply with the way it's being done in other places in this file, which is keeping it local.

[trasz]

Use NULL.

[trasz]

Move the code to a separate function.

[trasz]

Move stuff around.