command: allow merging stdout and stderr in string capture

Sometimes, its easier to run children with 2>&1 in shell notation,
and just deal with stdout and stderr interleaved.  This was already
possible for fd handling; extend it to also work when doing string
capture of a child process.

* docs/internals/command.html.in: Document this.
* src/util/command.c (virCommandSetErrorBuffer): Likewise.
(virCommandRun, virExecWithHook): Implement it.
* tests/commandtest.c (test14): Test it.
* daemon/remote.c (remoteDispatchAuthPolkit): Use new command
feature.

docs/internals/command.html.in

@@ -373,7 +373,10 @@
       allocation of collected information (however, on an
       out-of-memory condition, the buffer may still be NULL).  The
       caller is responsible for freeing registered buffers, since the
-      buffers are designed to persist beyond virCommandFree.
+      buffers are designed to persist beyond virCommandFree.  It
+      is possible to pass the same pointer to both
+      virCommandSetOutputBuffer and virCommandSetErrorBuffer, in which
+      case the child process interleaves output into a single string.
     </p>
 
     <h3><a name="directory">Setting working directory</a></h3>