command: allow merging stdout and stderr in string capture

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.