notify: Add EXIT_STATUS field

Whenever one of our tools or daemons exits, let's send the exit status
via sd-notify in the EXIT_STATUS field.

man/sd_notify.xml

@@ -170,6 +170,13 @@
         <literal>BUSERROR=org.freedesktop.DBus.Error.TimedOut</literal></para></listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term>EXIT_STATUS=…</term>
+
+        <listitem><para>If a service exits, the return value of its <function>main()</function> function.
+        </para></listitem>
+      </varlistentry>
+
       <varlistentry>
         <term>MAINPID=…</term>
 

src/shared/main-func.h

@@ -21,6 +21,7 @@
                 r = impl;                                               \
                 if (r < 0)                                              \
                         (void) sd_notifyf(0, "ERRNO=%i", -r);           \
+                (void) sd_notifyf(0, "EXIT_STATUS=%i", ret);            \
                 ask_password_agent_close();                             \
                 polkit_agent_close();                                   \
                 pager_close();                                          \