LookupServer+LibC: Add support for reverse DNS lookups via gethostbyaddr().

LookupServer can now take two types of requests:

* L: Lookup
* R: Reverse lookup

The /bin/host program now does a reverse lookup if the input string is a
valid IPv4 address. :^)

LibC/netdb.h

@@ -15,6 +15,7 @@ struct hostent {
 };
 
 struct hostent* gethostbyname(const char*);
+struct hostent* gethostbyaddr(const void* addr, socklen_t len, int type);
 
 struct servent {
     char* s_name;