Added ICMP Destination Unreachable codes from RFC-1812:

Added ICMP Destination Unreachable codes from RFC-1812:

   14 = Host Precedence Violation.  Sent by the first hop router to a
        host to indicate that a requested precedence is not permitted
        for the particular combination of source/destination host or

   15 = Precedence cutoff in effect.  The network operators have imposed
        a minimum level of precedence required for operation, the
        datagram was sent with a precedence below this level;