Fix pcap_loop() with a count of 0 and TPACKET_V3.

A count of -1 *or* 0 should mean "loop forever", so only a count > 0
means to use the count to limit the number of packets to process.
Add a PACKET_COUNT_IS_UNLIMITED() to test for a packet count <= 0.

In read routines, a packet count <= 0 means "keep supplying packets
until you run out of packets in the buffer", and it means "keep supply
packets until the loop is broken out of or you get an error" in
pcap_loop().

Use the macro in all tests for that, so the right test is always done
(i.e., a count of 0 means "unlimited", not "supply zero packets"); this
fixes some cases where we weren't doing the right test (and hopefully
encourages programmers to use it and get the test right in new modules).
Fix builds on systems without TPACKET_V3.