For RFC1918 private IPv4addresses, guess a better prefix length for
addresses and routes.
nmtui is an interactive program. It makes sense to be a bit smarter
about what the user probably meant.
It would be nice if nmtui would update the entry field immediately when
the cursor leaves the field, to show the guessed prefix length. However,
that is not easily possible, so lets to that another time.
For IPv6 addresses, default to /64 instead of /128.
https://bugzilla.redhat.com/show_bug.cgi?id=1474295