- PL2525 fix: implement SO_SETXID (reset the context ID of an existing

- PL2525 fix: implement SO_SETXID (reset the context ID of an existing
  socket). Callable by root context only. To avoid having to register a
  new sockopt number, #define SO_SETXID to be SO_PASSCRED (which is not
  usually callable by setsockopt()):

#if !defined(SO_SETXID)
#define SO_SETXID SO_PASSCRED
#endif

int xid;
setsockopt(fd, SOL_SOCKET, SO_SETXID, &xid, sizeof(xid));