passt: Fix build with gcc 7, use std=c99, enable some more Clang checkers

Unions and structs, you all have names now.

Take the chance to enable bugprone-reserved-identifier,
cert-dcl37-c, and cert-dcl51-cpp checkers in clang-tidy.

Provide a ffsl() weak declaration using gcc built-in.

Start reordering includes, but that's not enough for the
llvm-include-order checker yet.

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

tcp.h

@@ -39,7 +39,7 @@ union tcp_epoll_ref {
 				splice:1,
 				v6:1,
 				index:20;
-	};
+	} tcp;
 	uint32_t u32;
 };