init_main: Fix logging of the SYSINIT process

The current subsystem, aka sip->subsystem, should be logged, rather
than the last one.

Reviewed by: mhorne
Fixes: 5a8fceb3bd9f boottrace: trace annotations for startup and shutdown
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D47903

(cherry picked from commit 5997b1933142116041af7549eb2f450b4714b823)