Set nonblock for fd with mmap'ed read

With libpcap mmap'ed read on LInux, which would be the very
common case, even if we set pcap_setnonblock for that handle,
pcap_sendpacket will block.
For memory-mapped pcap_t's, set non-blocking mode on the FD first.

That way, if it fails, we don't set the timeout in the private data
structure to indicate that we're in non-blocking mode.