Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Update vfs_getopt(9) to reflect 32-bit to 64-bit change from
r224290.

PR: 159324
Submitted by: Brandon Gooch (jamesbrandongooch % gmail ! com)
Approved by: re (kib)

Extend the "vfsopt" mount options for more general use. Make struct
vfsopt and the vfs_buildopts function public, and add some new fields
to struct vfsopt (pos and seen), and new functions vfs_getopt_pos and
vfs_opterror.

Further extend the interface to allow reading options from the kernel
in addition to sending them to the kernel, with vfs_setopt and related
functions.

While this allows the "name=value" option interface to be used for more
than just FS mounts (planned use is for jails), it retains the current
"vfsopt" name and <sys/mount.h> requirement.

Approved by: bz (mentor)

Markup polishing.

Document the mount option handling functions.

Reviewed By: ru