Ensure TCP::OptionTypes has 8-bit range (mfontanini#259)

When reading TCP packets with esoteric (or corrupt) values for option types, the asan fsanitize=enum will trigger if the read value is not in range of the enum. The range of a classic (pre-C++11) enum with no negative enumerators is determined by the highest bit set in any of its enumerators, so if `TCP::OptionTypes` has highest enumerator `ALTCHK  = 14` it cannot take values above 15.

Define enumerators (per IANA) with bit 7 set to ensure that `TCP::OptionTypes` can take any 8-bit value.

An alternative (C++11 only) would be to give `TCP::OptionTypes` underlying type `uint8_t`.

include/tins/tcp.h

@@ -109,7 +109,9 @@ class TINS_API TCP : public PDU {
         SACK_OK = 4,
         SACK    = 5,
         TSOPT   = 8,
-        ALTCHK  = 14
+        ALTCHK  = 14,
+        RFC_EXPERIMENT_1 = 253,
+        RFC_EXPERIMENT_2 = 254
     };
 
     /**