pgsql: Use fork_process() to avoid some fork()-related boilerplate code

Log Message:
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Use fork_process() to avoid some fork()-related boilerplate code when
forking the stats collector child process.

Modified Files:
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pgsql/src/backend/postmaster:
pgstat.c (r1.89 -> r1.90)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/pgstat.c.diff?r1=1.89&r2=1.90)