log: Fix logging to stderr

This fix enables gluster clients like QEMU using glfs_set_logging()
with "-" as logfile to see client side errors on stderr.

Change-Id: I35f7b65460d1c03f910ceb3bad0a65677317e702
Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Reviewed-on: http://review.gluster.org/5637
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>