Overhaul of the vol_mgr.

Remove some new code that seems to give some race conditions.
This essentially restores the volume manager to the way it worked in 13.2.

Additionally we changed the following:
   - Cleanup comments
   - Make free_volume_list() a generic function.
        Instead of some trickery with using some temp vars give
        free_volume_list() two args e.g. a comment what about what its freeing
        and a dlist with the actual volume list it needs to free. Then we can
        reuse this in 3 places to delete any volume list. We also don't
        have to lock the vol_list when freeing the temp_vol_list as it
        now no longer "misuses" the global dlist pointer.

Issues #414: Bareos storage daemon crashes during backups

src/stored/reserve.c

@@ -504,6 +504,7 @@ bool find_suitable_device_for_job(JCR *jcr, RCTX &rctx)
 
       Dmsg0(dbglvl, "lock volumes\n");
       free_temp_vol_list(temp_vol_list);
+      temp_vol_list = NULL;
    }
    if (ok) {
       Dmsg1(dbglvl, "OK dev found. Vol=%s from in-use vols list\n", rctx.VolumeName);