pgsql: Fix unobvious interaction between -X switch and subdirectory cre

Fix unobvious interaction between -X switch and subdirectory creation.

Turns out the only reason initdb -X worked is that pg_mkdir_p won't
whine if you point it at something that's a symlink to a directory.
Otherwise, the attempt to create pg_xlog/ just like all the other
subdirectories would have failed.  Let's be a little more explicit
about what's happening.  Oversight in my patch for bug #13853
(mea culpa for not testing -X ...)

Branch
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REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/b05a34739fc1188056931eeec780817bdbb84381

Modified Files
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src/bin/initdb/initdb.c |   29 ++++++++++++++++++-----------
1 file changed, 18 insertions(+), 11 deletions(-)