<signal.h>/<sys/_siginfo.h>: Some fixes after my recent commits.

* In <sys/_siginfo.h>, include just <machine/stdint.h>. No more is
needed.

* In <signal.h>, continue to use struct __siginfo in st

<signal.h>/<sys/_siginfo.h>: Some fixes after my recent commits.

* In <sys/_siginfo.h>, include just <machine/stdint.h>. No more is
needed.

* In <signal.h>, continue to use struct __siginfo in struct sigaction,
primarily to deal with the case where __POSIX_VISIBLE is <199309,
of which for example graphics/png is one.

Reported-by: zrj

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Add <sys/_siginfo.h> to provide the definitions of siginfo_t and sigval.

They will be shared between <signal.h> and <sys/wait.h>.