Pass the length from the payload header to the print routines for

Pass the length from the payload header to the print routines for
various payload types.  Use that in those routines rather than getting
the length ourselves (which fixes a case where we were getting it
incorrectly, forgetting to "ntohs()" it).

In "isakmp_id_print()", if the item length is less than the size of the
fixed-length portion of the payload, don't subtract the latter from the
former and use it as the length of the variable-length portion, just set
the length in question to 0.