MINOR: quic: do not crash on unhandled sendto error

Remove ABORT_NOW() statement on unhandled sendto error. Instead use a
dedicated counter sendto_err_unknown to report these cases.

If we detect increment of this counter, strace can be used to detect
errno value :
  $ strace -p $(pidof haproxy) -f -e trace=sendto -Z

This should be backported up to 2.6.

This should help to debug github issue #1903.
diff --git a/src/quic_sock.c b/src/quic_sock.c
index 8617489..29f30cc 100644
--- a/src/quic_sock.c
+++ b/src/quic_sock.c
@@ -536,14 +536,14 @@
 	} while (ret < 0 && errno == EINTR);
 
 	if (ret < 0 || ret != sz) {
+		struct proxy *prx = qc->li->bind_conf->frontend;
+		struct quic_counters *prx_counters =
+		  EXTRA_COUNTERS_GET(prx->extra_counters_fe,
+		                     &quic_stats_module);
+
 		/* TODO adjust errno for UDP context. */
 		if (errno == EAGAIN || errno == EWOULDBLOCK ||
 		    errno == ENOTCONN || errno == EINPROGRESS || errno == EBADF) {
-			struct proxy *prx = qc->li->bind_conf->frontend;
-			struct quic_counters *prx_counters =
-			  EXTRA_COUNTERS_GET(prx->extra_counters_fe,
-			                     &quic_stats_module);
-
 			if (errno == EAGAIN || errno == EWOULDBLOCK)
 				HA_ATOMIC_INC(&prx_counters->socket_full);
 			else
@@ -551,7 +551,7 @@
 		}
 		else if (errno) {
 			/* TODO unlisted errno : handle it explicitly. */
-			ABORT_NOW();
+			HA_ATOMIC_INC(&prx_counters->sendto_err_unknown);
 		}
 
 		return 1;