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-gpg-verifier.h

@@ -75,4 +75,17 @@ void _ostree_gpg_verifier_add_keyring_file (OstreeGpgVerifier *self,
 void _ostree_gpg_verifier_add_key_ascii_file (OstreeGpgVerifier *self,
                                               const char        *path);
 
+gboolean
+_ostree_gpg_verifier_add_keyfile_path (OstreeGpgVerifier   *self,
+                                       const char          *path,
+                                       GCancellable        *cancellable,
+                                       GError             **error);
+
+gboolean
+_ostree_gpg_verifier_add_keyfile_dir_at (OstreeGpgVerifier   *self,
+                                         int                  dfd,
+                                         const char          *path,
+                                         GCancellable        *cancellable,
+                                         GError             **error);
+
 G_END_DECLS