Allow the caller to specify a separate output stream. stderr is always
a temporary file opened by runcmd(), so read from that to capture
output for error reporting *and* the return value.
The reasoning for the latter is a) that this preserves the traditional
behavior when out=None and b) if the caller wants the content of
stdout, it can read from the stream itself, which is not possible for
the temporary stderr.
Signed-off-by: Patrick Ohly <patrick.ohly@intel.com> Signed-off-by: Ross Burton <ross.burton@intel.com>
