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>

passt.h

@@ -51,8 +51,8 @@ union epoll_ref {
 			union udp_epoll_ref udp;
 			union icmp_epoll_ref icmp;
 			uint32_t data;
-		};
-	};
+		} p;
+	} r;
 	uint64_t u64;
 };