Add support for pidfd_send_signal

[chenxiaolong]

It looks like rustix already has support for the other two pidfd syscalls (pidfd_open and pidfd_getfd). Would it be possible to add support for pidfd_send_signal too?

int syscall(SYS_pidfd_send_signal, int pidfd, int sig,
            siginfo_t *_Nullable info, unsigned int flags);

Rustix has abstractions for signal numbers, but I'd imagine the unholy union that is siginfo_t might be a bit painful to wrap. It looks the only other siginfo_t API that's currently wrapped is waitid.