Handle volume restriction reads on volumes without quota backend

SYSCON reads volume/disk restrictions before opening the edit dialog.
For volumes that do not map to a quota-capable block device, such as
HOME on /home, the old code returned an error during the read path.
This made SYSCON abort with error -30312 before the user could even see
the restriction state.

Treat read requests on volumes without a quota backend as unrestricted:
return success with an unlimited quota and zero usage. Removing a
restriction on such a volume is also treated as a successful no-op.

Setting a non-zero restriction still fails, so admin tools do not report
a quota as enforced when no Linux quota backend exists.

src/nwvolume.c

@@ -982,7 +982,19 @@ int nw_set_vol_restrictions(uint8 volnr, int uid, uint32 quota)
   if (device == (char *) NULL) {
     XDPRINTF((2,0, "nw_set_vol_restrictions: no device for volume=%d path=%s",
               volnr, nw_volumes[volnr].unixname));
-    return(-0x98);
+
+    /* A volume without a quota-capable backing device is effectively
+     * unrestricted.  Removing/unsetting a restriction is therefore a
+     * successful no-op, but setting a real non-zero limit must fail so
+     * admin tools do not report an enforced quota where no backend exists.
+     */
+    if (quota == 0) {
+      XDPRINTF((2,0,
+        "nw_set_vol_restrictions: no quota device for volume=%d path=%s, remove/unlimited ignored",
+        volnr, nw_volumes[volnr].unixname));
+      return(0);
+    }
+    return(-0xfb);
   }
 
   /* If this call fails then it it probable that quotas are not enabled
@@ -1046,9 +1058,17 @@ int nw_get_vol_restrictions(uint8 volnr, int uid, uint32 *quota, uint32 *inuse)
 
   device=find_device_file(nw_volumes[volnr].unixname);
   if (device == (char *) NULL) {
-    XDPRINTF((2,0, "nw_get_vol_restrictions: no device for volume=%d path=%s",
-              volnr, nw_volumes[volnr].unixname));
-    return(-0x98);
+    /* SYSCON reads restrictions before opening the edit dialog.  Failing
+     * this read on HOME-style or pseudo volumes makes it abort with -30312.
+     * Report such volumes as unrestricted; actual non-zero Set/Change still
+     * fails in nw_set_vol_restrictions().
+     */
+    XDPRINTF((2,0,
+      "nw_get_vol_restrictions: no quota device for volume=%d path=%s, returning unlimited",
+      volnr, nw_volumes[volnr].unixname));
+    *quota = 0x40000000;
+    *inuse = 0;
+    return(0);
   }
 
   XDPRINTF((2,0, "Get quota for uid %d on device %s",