mkdirp: do not throw an error if directory exists

mkdirp: do not throw an error if directory exists

If I restart i3 4.10.2 twice, e.g. with

$ i3-msg restart; sleep 3; i3-msg restart

the second time I get the following two errors:

05/22/15 10:46:03 - ERROR: mkdir(/tmp/i3-theo.toAK7N) failed: File exists
05/22/15 10:46:03 - ERROR: Could not create "/tmp/i3-theo.toAK7N" for storing the restart layout, layout will be lost.

The first one is from mkdirp() in src/ipc.c and the second one is from
store_restart_layout() in src/util.c.

Notice that I do _not_ get the ``open()'' or ``Could not write restart layout to
...'' error messages, so the layout writing code after line 260 in
store_restart_layout() succeeded and the layout isn't actually lost.  Thus,
these error messages are a bit misleading, especially the second one (which is
triggered by the failure of mkdirp()).

POSIX says about `mkdir -p':

``Each dir operand that names an existing directory shall be ignored without
error.''

Therefore, I suggest the following simple patch that makes mkdirp() succeed if
the named file exists and actually is a directory.  This silences the second
error as well.