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
------
REL9_5_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/506ef1d07f8b51e9799f6856b03a4317297a39cb

Modified Files
--------------
src/bin/initdb/initdb.c | 37 ++++++++++++++++++++++++-------------
1 file changed, 24 insertions(+), 13 deletions(-)

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