boottrace: annotate init and shutdown utilities

[mhorne]

Summary

Add boottrace annotations to record events in init(8), shutdown(8), and
reboot(8). This is the first piece of the boottrace userland support.

[kevans]

sbin/init/init.c

1742

Declarations appearing mid-block are generally discouraged

[mhorne]

sbin/init/init.c

1742

Yeah. I think I kept this one here because it would be easier to overlook the static qualifier if it was declared at the start of the function. I wouldn't oppose increasing the scope and moving it to the top of the file.