sd-event: fix sd_event_source_get_inotify_path()

Follow-ups for 74c4231ce5.

Previously, the path is obtained from the fd, but it is closed in
sd_event_loop() to unpin the filesystem.
So, let's save the path when the event source is created, and make
sd_event_source_get_inotify_path() simply read it.

man/sd_event_add_inotify.xml

@@ -67,7 +67,7 @@
       <funcprototype>
         <funcdef>int <function>sd_event_source_get_inotify_path</function></funcdef>
         <paramdef>sd_event_source *<parameter>source</parameter></paramdef>
-        <paramdef>char **<parameter>ret</parameter></paramdef>
+        <paramdef>const char **<parameter>ret</parameter></paramdef>
       </funcprototype>
 
     </funcsynopsis>
@@ -144,7 +144,8 @@
     <para><function>sd_event_source_get_inotify_path()</function> retrieves the target path of the configured
     inotify watch of an event source created previously with <function>sd_event_add_inotify()</function>. It
     takes the event source object as the <parameter>source</parameter> parameter and a pointer to a
-    <type>char **</type> variable to return the path in. The caller needs to free the returned path.</para>
+    <type>const char **</type> variable to return the path in. The caller must not free the returned path.
+    </para>
   </refsect1>
 
   <refsect1>
@@ -207,8 +208,11 @@
         <varlistentry>
           <term><constant>-ENOSYS</constant></term>
 
-          <listitem><para><function>sd_event_add_inotify_fd()</function> was called without
-          <filename>/proc/</filename> mounted.</para></listitem>
+          <listitem>
+            <para><function>sd_event_add_inotify_fd()</function> or
+            <function>sd_event_source_get_inotify_path()</function> was called without
+            <filename>/proc/</filename> mounted.</para>
+          </listitem>
 
         </varlistentry>