Second try at port-limit option.

1) It's expected to fail to bind a new source port when they
   are scarce, suppress warning in log in this case.

2) Optimse bind_local when max_port - min_port is small. There's no
   randomness in this case, so we try all possible source ports
   rather than poking at random ones for an arbitrary number of tries.

3) In allocate_rfd() handle the case that all available source ports
   are already open. In this case we need to pick an existing
   socket/port to use, such that it has a different port from any we
   already hold. This gives the required property that the set of ports
   utilised by any given query is set by --port-limit and we don't
   re-use any until we have port-limit different ones.

src/dnsmasq.h

@@ -1405,6 +1405,7 @@ void *whine_malloc(size_t size);
 void *whine_realloc(void *ptr, size_t size);
 int sa_len(union mysockaddr *addr);
 int sockaddr_isequal(const union mysockaddr *s1, const union mysockaddr *s2);
+int sockaddr_isnull(const union mysockaddr *s);
 int hostname_order(const char *a, const char *b);
 int hostname_isequal(const char *a, const char *b);
 int hostname_issubdomain(char *a, char *b);