Emulate sigprocmask(), not sigsetmask(), on Windows.

Emulate sigprocmask(), not sigsetmask(), on Windows.

Since commit a65e0864, we've required Unix systems to have
sigprocmask().  As noted in that commit's message, we were still
emulating the historical pre-standard sigsetmask() function in our
Windows support code.  Emulate standard sigprocmask() instead, for
consistency.

The PG_SETMASK() abstraction is now redundant and all calls could in
theory be replaced by plain sigprocmask() calls, but that isn't done by
this commit.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/3153247.1657834482%40sss.pgh.pa.us