pgsql: initdb -S should now have an explicit check that $PGDATA is vali

initdb -S should now have an explicit check that $PGDATA is valid.

The fsync code from the backend essentially assumes that somebody's already
validated PGDATA, at least to the extent of it being a readable directory.
That's safe enough for initdb's normal code path too, but "initdb -S"
doesn't have any other processing at all that touches the target directory.
To have reasonable error-case behavior, add a pg_check_dir call.
Per gripe from Peter E.

Branch
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REL9_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/99f50dd720de59872bd842be3ec8eecea2be4b92

Modified Files
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src/bin/initdb/initdb.c |   11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)