pgsql: Fix incorrect mapping of fopen mode 'a' in recently-added code to

Log Message:
-----------
Fix incorrect mapping of fopen mode 'a' in recently-added code to
make fopen work safely on Windows. Magnus

Modified Files:
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pgsql/src/port:
open.c (r1.14 -> r1.15)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/port/open.c.diff?r1=1.14&r2=1.15)