repo: Add add-remotes-config-dir option

This option allows a repo to explicitly opt out of adding new remotes in
a remotes configuration directory. This currently defaults to true for
system repos and false for non-system repos to maintain legacy behavior
that non-system repos don't add remotes in a configuration directory.
That would be problematic for flatpak, which specifies a remotes config
dir but adds remotes in ways that are incompatible with it.

So, what this really does is allow system repos to control whether they
want to add remotes in the config dir or not. That's important if your
flatpak repo is the system repo like at Endless.

Closes: #1134

Closes: #1155
Approved by: cgwalters

man/ostree.repo-config.xml

@@ -122,6 +122,27 @@ Boston, MA 02111-1307, USA.
         keep free. The default value is 3.</para></listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><varname>add-remotes-config-dir</varname></term>
+        <listitem>
+          <para>
+            Boolean value controlling whether new remotes will be added
+            in the remotes configuration directory. Defaults to
+            <literal>true</literal> for system ostree repositories. When
+            this is <literal>false</literal>, remotes will be added in
+            the repository's <filename>config</filename> file.
+          </para>
+          <para>
+            This only applies to repositories that use a remotes
+            configuration directory such as system ostree repositories,
+            which use <filename>/etc/ostree/remotes.d</filename>.
+            Non-system repositories do not use a remotes configuration
+            directory unless one is specified when the repository is
+            opened.
+          </para>
+        </listitem>
+      </varlistentry>
+
     </variablelist>
   </refsect1>