Split registration of Win32 deadchild callback to separate function

Split registration of Win32 deadchild callback to separate function

The next commit will move the internal_forkexec() function to a
different source file, but it makes sense to keep all the code related
to the win32 waitpid() emulation in postmaster.c. Split it off to a
separate function now, to make the next commit more mechanical.

Reviewed-by: Tristan Partin, Andres Freund
Discussion: https://www.postgresql.org/message-id/7a59b073-5b5b-151e-7ed3-8b01ff7ce9ef@iki.fi