From Paolo Abeni and me: split pcap_open_live() into a "get a pcap_t

From Paolo Abeni and me: split pcap_open_live() into a "get a pcap_t
handle" routine, an 'activate a pcap_t handle" routine, and some "set
the properties of the pcap_t handle" routines, so that, for example, the
buffer size can be set on a BPF device before the device is bound to an
interface.

Add additional routines to set monitor mode, and make at least an
initial attempt at supporting that on Linux, *BSD, and Mac OS X 10.4 and
10.5.

Have a "timeout" member of the pcap_md structure on all platforms, use
that on Windows instead of the "timeout" member of the pcap_t structure,
and get rid of the "timeout" member of that structure.