pgsql: Fix race condition in win32 signal handling.

Log Message:
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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.

Modified Files:
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pgsql/src/backend/port/win32:
signal.c (r1.23 -> r1.24)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/port/win32/signal.c?r1=1.23&r2=1.24)