Remove #define _KERNEL hacks from libprocstat

Make sys/buf.h, sys/pipe.h, sys/fs/devfs/devfs*.h headers usable in
userspace, assuming that the consumer has an idea what it is for.
Unhide more material from sys/mount.h and sys/ufs/ufs/inode.h,
sys/ufs/ufs/ufsmount.h for consumption of userspace tools, with the
same caveat.

Remove unacceptable hack from usr.sbin/makefs which relied on sys/buf.h
being unusable in userspace, where it override struct buf with its own
definition. Instead, provide struct m_buf and struct m_vnode and adapt
code to use local variants.

Reviewed by: mckusick
Tested by: pho
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D28679