remote-add: Add --force option to add or replace remote

This uses the OSTREE_REPO_REMOTE_CHANGE_REPLACE operation to add a
remote or replace an existing one. This is roughly the opposite of
--if-not-exists and will raise an error if both options are passed.

Closes: #1166
Approved by: cgwalters

man/ostree-remote.xml

@@ -136,6 +136,14 @@ Boston, MA 02111-1307, USA.
                 </para></listitem>
             </varlistentry>
 
+            <varlistentry>
+                <term><option>--force</option></term>
+
+                <listitem><para>
+                    Replace the provided remote if it exists.
+                </para></listitem>
+            </varlistentry>
+
             <varlistentry>
                 <term><option>--no-gpg-verify</option></term>