Add APIs to put a "pcap_t" into or out of non-blocking mode, and to get

Add APIs to put a "pcap_t" into or out of non-blocking mode, and to get
the current state of non-blocking mode; this allows us to implement, for
example, memory-mapped capture devices, where "pcap_read()" uses
"select()" or "poll()" to wait for packets to arrive, and hide that
implementation detail from applications using this API
("pcap_setnonblock()" would set or clear a non-blocking mode flag in the
"pcap_t", and the "select()" or "poll()" would not be done if the
"pcap_t" is in non-blocking mode).