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