pgsql: Use dynamically-sized buffers in pgwin32_is_service().

Log Message:
-----------
Use dynamically-sized buffers in pgwin32_is_service().
Magnus Hagander

Modified Files:
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pgsql/src/backend/port/win32:
security.c (r1.6 -> r1.7)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/port/win32/security.c.diff?r1=1.6&r2=1.7)