syscalls: Add ioctl flags for the Hurd.

Use

    #include <stdio.h>
    #include <net/if.h>
    #include <hurd/ioctl.h>

    int
    main ()
    {
      printf ("SIOCSIFFLAGS #x%x\n", SIOCSIFFLAGS);
      printf ("SIOCGIFADDR #x%x\n", SIOCGIFADDR);
      printf ("SIOCSIFADDR #x%x\n", SIOCSIFADDR);
      printf ("SIOCGIFNETMASK #x%x\n", SIOCGIFNETMASK);
      printf ("SIOCSIFNETMASK #x%x\n", SIOCSIFNETMASK);
    #if 0
      printf ("SIOCADDRT #x%x\n", SIOCADDRT);
      printf ("SIOCDELRT #x%x\n", SIOCDELRT);
    #endif
    }

to fill in some blanks.  Adding and removing route apparently not supported.

* guix/build/syscalls.scm (SIOCSIFFLAGS SIOCGIFADDR, SIOCSIFADDR,
SIOCGIFNETMASK, SIOCSIFNETMASK):