Increase the size of the send and receive buffers for YP client rpc
calls to max allowed UDP datagram size.

Since max allowed size both for keys and values where increased, the
old sizes of around 1K cause ypmatch(3) failures, while plain maps
fetches work.

The buffers were reduced in r34146 from default UDP rpcclient values
to 1024/2304 due to the key and value size being 1K.

Reviewed by: slavash
Sponsored by: Mellanox Technologies
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D21586