package/nfs-utils: always use libtirpc and enable IPv6

package/nfs-utils: always use libtirpc and enable IPv6

nfs-utils selects rpcbind, and rpcbind unconditionally selects
libtirpc. Therefore, nfs-utils will never be used with the C library
RPC implementation: libtirpc will always be used. Consequently, all
the conditional logic to use libtirpc only if available is useless,
and we can use libtirpc unconditionally.

As an added bonus, this means that we can enable IPv6, because
libtirpc provides an IPv6-compatible RPC implementation.

Fixes: https://bugs.busybox.net/show_bug.cgi?id=10806
Signed-off-by: Carlos Santos <unixmania@gmail.com>
[Thomas: rework commit log]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>