Use the new debugging routines in libpcap.

Use the new debugging routines in libpcap.

Newer versions of libpcap, when configured to provide debugging
printouts for the filter expression parser or optimizer, provide
routines to set the "debug the parser" and "debug the optimizer" flags;
use them if the library has them.  If the library doesn't have them, do
our best to try to use the older mechanisms for requesting debugging, if
present.