Fix bugs in protocol file parsing

Protocol numbers were being used as a short in network byte order
instead of host byte order, so this command would fail:
nmap -sO -p tcp

Additionally, duplicate protocols would not be reported correctly. This
change speeds up lookups and simplifies the code.

protocols.h

@@ -75,18 +75,11 @@
 #include "libnetutil/netutil.h"
 #endif
 
-#define PROTOCOL_TABLE_SIZE 256
-
 struct nprotoent {
   const char *p_name;
   short p_proto;
 };
 
-struct protocol_list {
-  struct nprotoent *protoent;
-  struct protocol_list *next;
-};
-
 int addprotocolsfromservmask(char *mask, u8 *porttbl);
 struct nprotoent *nmap_getprotbynum(int num);