kboot: Add hostfs

Add hostfs for the Linux environment. We can't use the userboot one
that's kinda similar because the Linux system calls we have in kboot are
not quite POSIX compliant (Linux takes care of providing the POSIX
interface in libc), so we have to cope with a number of quirks in that
area.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D36607