Lazy umount emulation: deal with stopped volumes

On non Linux systems, lazy umount is emulated using contrib/umountd.
It first check that the path given to unmount exists, but it should
not give up on ENOTCONN as it is what happens when a volume is mounted
but stopped.

This lets NetBSD pass tests/bugs/bug-1049323.t

BUG: 1129939
Change-Id: I3451362453607a0fd82b095a9e5aa6f63bfe869a
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/8991
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>