Don't return NXDOMAIN to empty non-terminals.

When a record is defined locally, eg an A record for one.two.example then
we already know that if we forward, eg an AAAA query for one.two.example,
and get back NXDOMAIN, then we need to alter that to NODATA. This is handled
by  check_for_local_domain(). But, if we forward two.example, because
one.two.example exists, then the answer to two.example should also be
a NODATA.

For most local records this is easy, just to substring matching.
for A, AAAA and CNAME records that are in the cache, it's more difficult.
The cache has no efficient way to find such records. The fix is to
insert empty (none of F_IPV4, F_IPV6 F_CNAME set) records for each
non-terminal.

The same considerations apply in auth mode, and the same basic mechanism
is used there too.

src/dnsmasq.h

@@ -1135,6 +1135,7 @@ void next_uid(struct crec *crecp);
 void log_query(unsigned int flags, char *name, struct all_addr *addr, char *arg); 
 char *record_source(unsigned int index);
 char *querystr(char *desc, unsigned short type);
+int cache_find_non_terminal(char *name, time_t now);
 struct crec *cache_find_by_addr(struct crec *crecp,
 				struct all_addr *addr, time_t now, 
 				unsigned int prot);
@@ -1250,6 +1251,7 @@ void *whine_malloc(size_t size);
 int sa_len(union mysockaddr *addr);
 int sockaddr_isequal(union mysockaddr *s1, union mysockaddr *s2);
 int hostname_isequal(const char *a, const char *b);
+int hostname_issubdomain(char *a, char *b);
 time_t dnsmasq_time(void);
 int netmask_length(struct in_addr mask);
 int is_same_net(struct in_addr a, struct in_addr b, struct in_addr mask);