SYSINIT: add SI_SUB_KENV

[glebius]

Summary

Since the kernel environment has its own dependencies, lurking at the end
of the SI_SUB_KMEM sequence appeared to be fragile. Provide own subsystem
for it. The init_dynamic_kenv() goes SI_ORDER_FIRST, and two modules that
depend on it go SI_ORDER_ANY.

PR: 297492

[agh_riseup.net]

Works here on a custom host at -CURRENT.

Thanks.