lib, fish: Handle Gluster, NBD, iSCSI and Sheepdog paths properly.

This fixes the handling of paths for the protocols named above, with
respect to leading '/' (or not) on the volume/export name.

See previous commits which did the same fixes for Ceph:
commit 53a3ff9c005fd69e773fdb18461a635c2502df1b
commit 992a6b297023711862229aabe4e23b6a1c94c1cc