Rename the "private" member of a pcap_t to avoid C++ issues.

"private" is a C++ keyword; rename the "private" member of a pcap_t to
"priv" to avoid that, as per Gisle Vanem's suggestion.
Declare pcap_get_adapter() on Win32.
Add stream support to dag_create()

DAG capture device names are of the format dagN or dagN:M where N
is the device number and M is the stream number. The former implies
stream 0.
Merge pull request #313 from sfd/master

Fix DAG Stream support in dag_create()