mountd: Convert colons in IPv6 presentation addresses tosemicolons

The /var/lib/nfs/rmtab file delineates fields in each of its lines
with a ":".  The first field contains the IP address of a client, in
presentation format.  IPv6 presentation format addresses contain
colons, which screws up the field delineation of rmtab.

Use a simple simple scheme to convert the colons in incoming client
names to some other character, and then convert them back when the
rmtab file is read.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>