Add support for recvmmsg().

Add support for recvmmsg().

Performance analysis with Linux's perf tool has shown that while
transferring a stream of data, MeshLink's main thread spends 66% of the
time in the system calls select(), sendto() and recvfrom() combined. One
way to reduce that is to handle more than one packet at once with a
single system call.

This commit makes MeshLink use recvmmsg() if available.