init_main: Switch from sysinit array to SLIST

init_main: Switch from sysinit array to SLIST

This has two effects:
1. We can mergesort the sysinits instead of bubblesorting them, which
shaves about 2 ms off the boot time in Firecracker.
2. Adding more sysinits (e.g. from a KLD) can be performed by sorting
them and then merging the sorted lists, which is both faster than
the previous "append and sort" approach and avoids needing malloc.

Reviewed by:	jhb (previous version)
Sponsored by:	https://www.patreon.com/cperciva
Differential Revision:	https://reviews.freebsd.org/D41075