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)
