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
c94ae9d827