passt: Introduce packet capture implementation

With -DDEBUG, passt now saves guest-side traffic captures in
pcap format at /tmp/passt_<ISO8601 timestamp>.pcap. The timestamp
refers to time and date of start-up.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

pcap.h

@@ -0,0 +1,2 @@
+void pcap(char *pkt, size_t len);
+void pcap_init(void);