busctl: add introspect --xml-interface

This wraps the call to org.freedesktop.DBus.Introspectable.Introspect.
Using "busctl call" directly is inconvenient because busctl escapes the
string before printing.

Example:
$ busctl introspect --xml org.freedesktop.systemd1 /org/freedesktop/systemd1 | pygmentize -lxml | less -RF

man/busctl.xml

@@ -140,6 +140,16 @@
         </listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><option>--xml-interface</option></term>
+
+        <listitem>
+          <para>When used with the <command>introspect</command> call, dump the XML description received from
+          the D-Bus <constant>org.freedesktop.DBus.Introspectable.Introspect</constant> call instead of the
+          normal output.</para>
+        </listitem>
+      </varlistentry>
+
       <varlistentry>
         <term><option>--json=</option><replaceable>MODE</replaceable></term>