daemon: use ntfslabel(1) to get ntfs labels

blkid(1) (or actually, libblkid) seems to handle filesystem labels up
to 127 characters. Considering that btrfs labels can be up to 128
characters, this means long labels are not read correctly (i.e. get
truncated) by blkid.  Furthermore, ntfs labels are actually unicode,
and libblkid seems to not decode them correctly.

Hence, if ntfsprogs is available invoke `ntfslabel` to get the label
of ntfs filesystems.

Related to RHBZ#1164708.

daemon/blkid.c

@@ -82,6 +82,8 @@ do_vfs_label (const mountable_t *mountable)
   if (type) {
     if (STREQ (type, "btrfs") && optgroup_btrfs_available ())
       return btrfs_get_label (mountable->device);
+    if (STREQ (type, "ntfs") && optgroup_ntfsprogs_available ())
+      return ntfs_get_label (mountable->device);
   }
 
   return get_blkid_tag (mountable->device, "LABEL");

daemon/daemon.h

@@ -265,6 +265,9 @@ extern char *debug_bmap_device (const char *subcmd, size_t argc, char *const *co
 /*-- in btrfs.c --*/
 extern char *btrfs_get_label (const char *device);
 
+/*-- in ntfs.c --*/
+extern char *ntfs_get_label (const char *device);
+
 /* ordinary daemon functions use these to indicate errors
  * NB: you don't need to prefix the string with the current command,
  * it is added automatically by the client-side RPC stubs.

daemon/ntfs.c

@@ -33,6 +33,7 @@
 GUESTFSD_EXT_CMD(str_ntfs3g_probe, ntfs-3g.probe);
 GUESTFSD_EXT_CMD(str_ntfsresize, ntfsresize);
 GUESTFSD_EXT_CMD(str_ntfsfix, ntfsfix);
+GUESTFSD_EXT_CMD(str_ntfslabel, ntfslabel);
 
 int
 optgroup_ntfs3g_available (void)
@@ -46,6 +47,29 @@ optgroup_ntfsprogs_available (void)
   return prog_exists (str_ntfsresize);
 }
 
+char *
+ntfs_get_label (const char *device)
+{
+  int r;
+  CLEANUP_FREE char *err = NULL;
+  char *out = NULL;
+  size_t len;
+
+  r = command (&out, &err, str_ntfslabel, device, NULL);
+  if (r == -1) {
+    reply_with_error ("%s", err);
+    free (out);
+    return NULL;
+  }
+
+  /* Trim trailing \n if present. */
+  len = strlen (out);
+  if (len > 0 && out[len-1] == '\n')
+    out[len-1] = '\0';
+
+  return out;
+}
+
 int
 do_ntfs_3g_probe (int rw, const char *device)
 {