lib/repo: Search a list of paths in gpgkeypath for gpg keys

This allows specifying gpgpath as list of
paths that can point to a file or a directory. If a directory path
is given, paths to all regular files in the directory are added
to the remote as gpg ascii keys. If the path is not a directory,
the file is directly added (whether regular file, empty - errors
will be reported later when verifying gpg keys e.g. when pulling).

Adding the gpgkeypath property looks like:

ostree --repo=repo remote add --set=gpgpath="/path/key1.asc,/path/keys.d" R1 https://example.com/some/remote/ostree/repo

Closes #773

Closes: #1773
Approved by: cgwalters

src/libostree/ostree-repo.c

@@ -5081,7 +5081,6 @@ _ostree_repo_gpg_verify_data_internal (OstreeRepo    *self,
     }
   else if (remote_name != NULL)
     {
-      g_autofree char *gpgkeypath = NULL;
       /* Add the remote's keyring file if it exists. */
 
       g_autoptr(OstreeRemote) remote = NULL;
@@ -5100,12 +5099,19 @@ _ostree_repo_gpg_verify_data_internal (OstreeRepo    *self,
           add_global_keyring_dir = FALSE;
         }
 
-      if (!ot_keyfile_get_value_with_default (remote->options, remote->group, "gpgkeypath", NULL,
-                                              &gpgkeypath, error))
+      g_auto(GStrv) gpgkeypath_list = NULL;
+
+      if (!ot_keyfile_get_string_as_list (remote->options, remote->group, "gpgkeypath",
+                                          ";,", &gpgkeypath_list, error))
         return NULL;
 
-      if (gpgkeypath)
-        _ostree_gpg_verifier_add_key_ascii_file (verifier, gpgkeypath);
+      if (gpgkeypath_list)
+        {
+          for (char **iter = gpgkeypath_list; *iter != NULL; ++iter)
+            if (!_ostree_gpg_verifier_add_keyfile_path (verifier, *iter,
+                                                        cancellable, error))
+              return NULL;
+        }
     }
 
   if (add_global_keyring_dir)