With DECC$VALIDATE_SIGNAL_IN_KILL enabled, a signal value that
    is in the range 0 to _SIG_MAX but is not supported by the C RTL
    generates an error with errno set to EINVAL, which makes the
    behavior the same as for raise.

    With this logical name disabled, validation of signals is
    restricted to checking that the signal value is in the range 0 to
    _SIG_MAX. If sys$sigprc fails, errno is set based on sys$sigprc
    exit status.