pgsql: Use AllocateFile(), FreeFile() and palloc() rather than fopen(),

Log Message:
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Use AllocateFile(), FreeFile() and palloc() rather than fopen(), fclose()
and malloc() in pgstat.c, respectively. This simplifies error recovery,
as well as being more consistent with the rest of the backend.

Modified Files:
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pgsql/src/backend/postmaster:
pgstat.c (r1.83 -> r1.84)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/pgstat.c.diff?r1=1.83&r2=1.84)