Use the new libpcap <pcap/nflog.h> for NFLOG definitions and declarations.

Use the new libpcap <pcap/nflog.h> for NFLOG definitions and declarations.

Use the header file for information about LINKTYPE_NFLOG files.

Don't build the NFLOG printer if we don't have that header file.

Fix some checks for pcap/XXX.h header files to look for
"tcpdump-stdinc.h" rather than <tcpdump-stdinc.h>, so that we don't fail
to find that header.