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)
Approved by: re (gjb)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D41749

(cherry picked from commit 1926d5cef6eaa1302024d6471271c930ad65f9b2)
(cherry picked from commit c31f2b5e6b591c61711018c3d6e52067ae860e9c)