util: replace strerror/strerror_r with g_strerror

g_strerror is offers the safety/correctness benefits of strerror_r, with
the API design convenience of strerror.

Use of virStrerror should be eliminated through the codebase in favour
of g_strerror.

commandhelper.c is a special case as its a tiny single threaded test
program, not linked to glib, so it just uses traditional strerror().

Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

docs/hacking.html.in

@@ -1051,6 +1051,10 @@ BAD:
       <dd>The GLib macros g_autoptr and G_DEFINE_AUTOPTR_CLEANUP_FUNC
         should be used to manage autoclean of virObject classes.
         This matches usage with GObject classes.</dd>
+
+      <dt>virStrerror</dt>
+      <dd>The GLib g_strerror() function should be used instead,
+        which has a simpler calling convention as an added benefit.</dd>
     </dl>
 
     <h2><a id="file_handling">File handling</a></h2>