add Siginfo_t

add Siginfo_t

From the code comments:

    This is an alias for the OS's siginfo_t, except that where the OS
    doesn't support it, declare a dummy version instead. This allows us to
    have signal handler functions which always have a Siginfo_t parameter
    regardless of platform, (and which will just be passed a NULL value
    where the OS doesn't support HAS_SIGACTION).

It doesn't actually do anything useful yet, but will shortly allow
signal handler functions to be rationalised.