Use FQDN_LEN instead of MAXHOSTNAMELEN for DNS name buffers

This closes #140 (issue #140), namely "Use correct lengths 
for FQDN, not MAXHOSTNAMELEN"

nmap.h

@@ -323,6 +323,9 @@
 #define MAXHOSTNAMELEN 64
 #endif
 
+/* Length of longest DNS name */
+#define FQDN_LEN 254
+
 /* Max payload: Worst case is IPv4 with 40bytes of options and TCP with 20
  * bytes of options. */
 #define MAX_PAYLOAD_ALLOWED 65535-60-40