make pcap_setdirection() call conditional, GH #252

pcap_setdirection() succeeds only for particular combination of
platform, interface type and direction. Calling it on each invocation
(with PCAP_D_INOUT by default) was wrong: some interfaces are two-way,
others are one-way, and pcap_setdirection() has its own idea what it
supports and what doesn't. In particular, it was impossible to do any
capture on nflog interface in Linux since commit df7d24f. Now the
function is only called on -P flag.