Provide sigaction() for Windows.
Commit
9abb2bfc left behind code to block signals inside signal
handlers on Windows, because our signal porting layer didn't have
sigaction(). Provide a minimal implementation that is capable of
blocking signals, to get rid of platform differences. See also related
commit
c94ae9d8.
Discussion: https://postgr.es/m/CA%2BhUKGKKKfcgx6jzok9AYenp2TNti_tfs8FMoJpL8%2B0Gsy%3D%3D_A%40mail.gmail.com