DragonFlyBSD

Hoi,

this patch adds an 'ignore' option to mount.
The actual infrustructure is already in place (MNT_IGNORE)
but it can't be set.
This means df(1) already has code to ignore MNT_IGNORE mounts
and an option to also show ignored mounts (-a), but the user
currently can't set the ignored option.
Below patch fixes that.
This is useful for procfs, amd, nullfs, filebacked fs, etc.
To test it with procfs you also need to recompile mount_std(8).
Use a fstab entry like:
proc /proc procfs rw,ignore 0 0

Some systems call the option 'hidden', but i don't think it
is semantically any better (both aren't very good), but at least
'ignore' corresponds to the MNT_IGNORE flag.