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

Backport of: http://review.gluster.org/8991

BUG: 1138897
Change-Id: I39941577021c0b39f545f9777fe75cd39637427b
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/9006
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>