Add checks for NFS overlapping networks

When creating complex NFS shares the freenas ui will not let you create overlapping NFS share settings for the same export.

For example:

export /mnt/tank/export to all machines on the subnet 192.168.0.0/24 as read only.

export /mnt/tank/export to administrator.localdomain (192,168.0.2) on the same subnet as read/write.

When doing this from the ui you are presented with a error stating you cannot overlap the NFS shares.

Generic error from the ui:

The network and host must be unique per share and per filesystem or
directory. Since :file:`/etc/exports` does not act like an ACL, the
rule to apply is undefined among overlapping networks or when using
the same share with multiple hosts.

Now if we manually create the shares ourselves it works without issue applying the correct permissions.

If we check the man page for freebsd exports : https://www.freebsd.org/cgi/man.cgi?exports(5)

We are presented with examples showing this exact usage:

/u2 -maproot=root friends
/u2 -alldirs -network cis-net -mask cis-mask
The file system rooted at
/u2
is exported to the hosts in ``friends '' with root mapped to UID and groups associated with ``root' '; it is exported to all hosts on network ``cis-net' ' allowing mounts at any directory within /u2.

Things means the ui would need to be changed to allow overlapping networks in NFS shares to allow more complex sharing without having to manually edit the exports file.