logging: use duplicate stderr, instead of re-opening

The special filename "-" is supposed to log to stderr. Instead of
trying to explictly open "/dev/stderr" again (which may not be possible
as permissions might have changed by then), dup the stderr and use
the copy.

It is not a good idea to use @stderr global variable directly, as
ctx->log.logfile is fclose()d in glfs_fini() (was fixed in
http://review.gluster.org/6452)

Change-Id: Ia6c538fe363905588dcf4fc4783804073956a586
BUG: 1088589
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/7607
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>