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>

checksum.c

@@ -46,12 +46,11 @@
  * See the comment to csum_avx2() for further details.
  */
 
-#include <stdint.h>
-#include <stddef.h>
-#include <linux/ip.h>
-#include <linux/tcp.h>
-#include <linux/udp.h>
 #include <arpa/inet.h>
+#include <netinet/ip.h>
+#include <netinet/tcp.h>
+#include <stddef.h>
+#include <stdint.h>
 
 /**
  * sum_16b() - Calculate sum of 16-bit words