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

(cherry picked from commit 02dba4f75f86f3d5ae5a6a438b5bcfdc667929bb)