man: recommend strerror_r() over strerror()

Let's nudge people towards the use of an anonymous buffer like we
do internally.

"errno" → "errnum", to match the man page for strerror, and also to avoid
confusion with the global variable. In general, I think that errno is a
terrible interface and we shouldn't encourage people to use it. Those functions
use errno-style error numbers, which are a different thing.

man/sd-bus-errors.xml

@@ -268,7 +268,7 @@
       <citerefentry><refentrytitle>sd_bus_error</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
       <citerefentry><refentrytitle>sd_bus_message_set_allow_interactive_authorization</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
       <citerefentry project='man-pages'><refentrytitle>errno</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
-      <citerefentry project='die-net'><refentrytitle>strerror</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+      <citerefentry project='die-net'><refentrytitle>strerror_r</refentrytitle><manvolnum>3</manvolnum></citerefentry>
     </para>
   </refsect1>
 

man/sd_notify.xml

@@ -424,8 +424,8 @@
 
       <programlisting>sd_notifyf(0, "STATUS=Failed to start up: %s\n"
         "ERRNO=%i",
-        strerror(errno),
-        errno);</programlisting>
+        strerror_r(errnum, (char[1024]){}, 1024),
+        errnum);</programlisting>
     </example>
 
     <example>

src/systemd/sd-daemon.h

@@ -260,8 +260,8 @@ int sd_notify(int unset_environment, const char *state);
 
      sd_notifyf(0, "STATUS=Failed to start up: %s\n"
                    "ERRNO=%i",
-                   strerror(errno),
-                   errno);
+                   strerror_r(errnum, (char[1024]){}, 1024),
+                   errnum);
 
   See sd_notifyf(3) for more information.
 */