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>

qrap.c

@@ -19,14 +19,14 @@
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <errno.h>
-#include <linux/if_ether.h>
-#include <linux/ipv6.h>
 #include <linux/limits.h>
-#include <linux/un.h>
 #include <limits.h>
 #include <fcntl.h>
 #include <net/if_arp.h>
 #include <netinet/in.h>
+#include <netinet/if_ether.h>
+
+#include <linux/ipv6.h>
 
 #include "util.h"
 #include "passt.h"