If "-null" is given as the map it is used to tell automount(8) to
ignore a subsequent master map entry with the given path.
It can only be used for paths that appear in the master map (or
in direct mount maps).
An indirect mount map top level mount point path can be nulled.
If so no mounts from the nulled mount are performed (essentially
it isn't mounted).
Direct mount map path entries can be nulled. Since they must be
present at startup they are (notionally) part of the master map.
A nulled master map entry path will ignore a single subsequent
matching entry. Any matching entry following that will be treated
as it normally would be. An example use of this is allowing local
master map entries to override remote ones.
NOTE: If a duplicate master map entry path is seen (excluding
paths of null entries) it will be ignored and noted in the log,
that is the first encountered master map entry is used unless
there is a corresponding null entry.
