Allows for determining if a buffer large enough for multiple dgrams
should be allocated in alloc_cb of uv_udp_recvstart, for example.
Contributes towards #2822.
PR-URL: https://github.com/libuv/libuv/pull/2830
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Jameson Nash <vtjnash@gmail.com>