Fix FTBFS when using -pedantic compiler flag.

I am attaching an incremental git-am ready patch to go on top your Git HEAD,
to fix all sorts of issues and make this conforming C99 with default
options set,
and fix another load of warnings you receive when setting the compiler
to pick the nits,
-pedantic-errors -std=c99 (or c11, c18, c2x).
It changes many void * to uint8_t * to make the "increment by bytes"
explicit.
You can't do:

void *foo;
// ...
foo += 2.

src/arp.c

@@ -35,7 +35,7 @@ struct arp_record {
 static struct arp_record *arps = NULL, *old = NULL, *freelist = NULL;
 static time_t last = 0;
 
-static int filter_mac(int family, char *addrp, char *mac, size_t maclen, void *parmv)
+static int filter_mac(int family, void *addrp, char *mac, size_t maclen, void *parmv)
 {
   struct arp_record *arp;