Do not hardcode umount(8) path, emulate lazy umount

1) Use a system-dependent macro for umount(8) location instead of
relying on $PATH  to find it, for security and portability sake.

2) Introduce gf_umount_lazy() to replace umount -l (-l for lazy) invocations,
which is only supported on Linux; On Linux behavior in unchanged. On other
systems, we fork an external process (umountd) that will take care of
periodically attempt to unmount, and optionally rmdir.

BUG: 1129939
Change-Id: Ia91167c0652f8ddab85136324b08f87c5ac1e51d
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/8649
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Csaba Henk <csaba@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>