init_main: Switch from SLIST to STAILQ, fix order

[cperciva]

Summary

Constructing an SLIST of SYSINITs by inserting them one by one at the
head of the list resulted in them being sorted in anti-stable order:
When two SYSINITs tied for (subsystem, order), they were executed in
the reverse order to the order in which they appeared in the linker
set.

Fixes: 9a7add6d01f3 "init_main: Switch from sysinit array to SLIST"

[gallatin]

Thank you; this fixes the weird performance problem that we've yet to fully root cause.