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.

(cherry picked from commit 92aebdeaff57091c77398912accb4c758c6a0fcb)