Fix DAG Stream support in dag_create()

It appears that when "Have non-interface modules take responsibility for identifying their devices" https://github.com/the-tcpdump-group/libpcap/commit/2426611584e9099af5f98d18ef37337df9bef025 was committed, the heuristic for DAG device names was insufficient.

The impact is that libpcap cannot capture from DAG streams other than 0, which will significantly affect some users.

This issue also affects libpcap_1.4 branch.

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.

Thanks to joseph.ciccone@emulex.com for his initial patch.