passt: Spare some syscalls, add some optimisations from profiling

Avoid a bunch of syscalls on forwarding paths by:

- storing minimum and maximum file descriptor numbers for each
  protocol, fall back to SO_PROTOCOL query only on overlaps

- allocating a larger receive buffer -- this can result in more
  coalesced packets than sendmmsg() can take (UIO_MAXIOV, i.e. 1024),
  so make sure we don't exceed that within a single call to protocol
  tap handlers

- nesting the handling loop in tap_handler() in the receive loop,
  so that we have better chances of filling our receive buffer in
  fewer calls

- skipping the recvfrom() in the UDP handler on EPOLLERR -- there's
  nothing to be done in that case

and while at it:

- restore the 20ms timer interval for periodic (TCP) events, I
  accidentally changed that to 100ms in an earlier commit

- attempt using SO_ZEROCOPY for UDP -- if it's not available,
  sendmmsg() will succeed anyway

- fix the handling of the status code from sendmmsg(), if it fails,
  we'll try to discard the first message, hence return 1 from the
  UDP handler

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

udp.h

@@ -1,4 +1,19 @@
+#ifndef UDP_H
+#define UDP_H
+
 void udp_sock_handler(struct ctx *c, int s, uint32_t events);
 int udp_tap_handler(struct ctx *c, int af, void *addr,
 		    struct tap_msg *msg, int count);
 int udp_sock_init(struct ctx *c);
+
+/**
+ * struct udp_ctx - Execution context for UDP
+ * @fd_min:		Lowest file descriptor number for UDP ever used
+ * @fd_max:		Highest file descriptor number for UDP ever used
+ */
+struct udp_ctx {
+	int fd_min;
+	int fd_max;
+};
+
+#endif /* UDP_H */