mount(8): Add an 'ignore' option.

mount(8): Add an 'ignore' option.

The actual infrustructure was already in place (MNT_IGNORE) but it
couldn't be set, i.e. df(1) already had code to ignore MNT_IGNORE
mounts and an option to also show ignored mounts (-a), but the user
couldn't set the ignored option.

This is useful for procfs, amd, nullfs, filebacked fs, etc.
To test it with procfs you also need to recompile mount_std(8).

Some systems call the option 'hidden', but it doesn't seem any better
(both aren't very good), but at least 'ignore' corresponds to
MNT_IGNORE.

Submitted-by:  Andreas Hauser <andy@splashground.de>
Dragonfly-bug: <http://bugs.dragonflybsd.org/issue200>