introduce WITH_POSIX (as compared to WITH_COAP etc)

this will make reading the ifdefed sections easier.

as the typedef struct __coap_address_t {...}; /
define coap_address_t __coap_address_t trick is not needed any more to
detect whether a WITH defined coap_address_t or not, it was removed.

debug.c

@@ -137,7 +137,7 @@ print_readable( const unsigned char *data, unsigned int len,
 #endif
 
 size_t
-coap_print_addr(const struct __coap_address_t *addr, unsigned char *buf, size_t len) {
+coap_print_addr(const struct coap_address_t *addr, unsigned char *buf, size_t len) {
 #ifdef HAVE_ARPA_INET_H
   const void *addrptr = NULL;
   in_port_t port;