inetd: free WITHOUT_INET6_SUPPORT build of warnings

If inetd is compiled without inet6 support, we need to error out on
rpc+inet6 services rather than attempting to call into rpc bits with an
uninitialized netid.

v4bind is only used with INET6 support, so move it under the proper #ifdefs
with v6bind.

Reported by: Pavel Timofeev <timp87 gmail com>
MFC after: 3 days