init_main: Record completed SYSINITs

When removing them from sysinit_list, append them to sysinit_done_list;
print this list from 'show sysinit' along with the list of future
sysinits.

Reviewed by: jhb, gallatin (previous version)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D41749