mountd: deprecate exports to a network without mask

The exports file format allows export to a network using an explicit
mask or prefix length (CIDR). It also allows a network with just
a dotted address, in which case the historical mask was used.
Deprecate this usage, and warn when it is used. Document that this
is deprecated.

MFC after: 1 month
Reviewed by: rmacklem, bcr, manpages
Differential Revision: https://reviews.freebsd.org/D32713