pgsql: Repair longstanding bug in slru/clog logic: it is possible for

Log Message:
-----------
Repair longstanding bug in slru/clog logic: it is possible for two backends
to try to create a log segment file concurrently, but the code erroneously
specified O_EXCL to open(), resulting in a needless failure. Before 7.4,
it was even a PANIC condition :-(. Correct code is actually simpler than
what we had, because we can just say O_CREAT to start with and not need a
second open() call. I believe this accounts for several recent reports of
hard-to-reproduce "could not create file ...: File exists" errors in both
pg_clog and pg_subtrans.

Modified Files:
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pgsql/src/backend/access/transam:
slru.c (r1.33 -> r1.34)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/slru.c.diff?r1=1.33&r2=1.34)