passt: Add cppcheck target, test, and address resulting warnings

...mostly false positives, but a number of very relevant ones too,
in tcp_get_sndbuf(), tcp_conn_from_tap(), and siphash PREAMBLE().

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

1 files changed, 1 insertions, 1 deletions

diff --git a/passt.h b/passt.h
index 68c42ca..ae3035f 100644
--- a/passt.h
+++ b/passt.h
@@ -44,7 +44,7 @@ union epoll_ref;
  */
 union epoll_ref {
 	struct {
-		uint32_t	proto:8,
+		int32_t		proto:8,
 				s:24;
 		union {
 			union tcp_epoll_ref tcp;