Handle using cooked mode for DLT_NETLINK in activate_new().

That's how we handle it elsewhere; if that can be cleaned up, especially
if we can avoid opening a raw socket in the cooked-mode case, that would
be nice.

Note this in comments.
Merge branch 'master' of git+ssh://bpf.tcpdump.org/tcpdump/master/git/libpcap

This merge is necessary because this isn't SVN and "commit" and "push to
official repository" are separate operations, so, when you do a push, it
apparently needs a commit to merge your changes with other changes.
Do this the way DLT_LINUX_IRDA and DLT_LINUX_LAPD are handled.

I'm not certain setting handlep->cooked in map_arphrd_to_dlt() will
work.  We should look at making that work.

Add comments to indicate what we're doing.