pgsql: Fix race condition in win32 signal handling.

Log Message:
-----------
Fix race condition in win32 signal handling.

There was a race condition where the receiving pipe could be closed by the
child thread if the main thread was pre-empted before it got a chance to
create a new one, and the dispatch thread ran to completion during that time.

One symptom of this is that rows in pg_listener could be dropped under
heavy load.

Analysis and original patch by Radu Ilie, with some small
modifications by Magnus Hagander.

Tags:
----
REL8_3_STABLE

Modified Files:
--------------
pgsql/src/backend/port/win32:
signal.c (r1.20 -> r1.20.2.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/port/win32/signal.c?r1=1.20&r2=1.20.2.1)