udp, passt: Introduce socket packet buffer, avoid getsockname() for UDP

This is in preparation for scatter-gather IO on the UDP receive path:
save a getsockname() syscall by setting a flag if we get the numbering
of all bound sockets in a strict sequence (expected, in practice) and
repurpose the tap buffer to be also a socket receive buffer, passing
it down to protocol handlers.

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

icmp.h

@@ -3,7 +3,7 @@
 
 struct ctx;
 
-void icmp_sock_handler(struct ctx *c, int s, uint32_t events,
+void icmp_sock_handler(struct ctx *c, int s, uint32_t events, char *pkt_buf,
 		       struct timespec *now);
 int icmp_tap_handler(struct ctx *c, int af, void *addr,
 		     struct tap_msg *msg, int count, struct timespec *now);