pgsql: Provide sigaction() for Windows.

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b28ac1d24db44ae763a57688e7a559352002badc

Modified Files
--------------
src/backend/libpq/pqsignal.c        |  9 ------
src/backend/port/win32/signal.c     | 41 ++++++++++++++++++--------
src/backend/postmaster/postmaster.c | 57 +++----------------------------------
src/include/libpq/pqsignal.h        | 14 +++++++++
src/port/pqsignal.c                 | 11 +++----
5 files changed, 53 insertions(+), 79 deletions(-)