Add a test case to commandtest.c to test the transfer of data to a
process who received the read-end of pipes' file descriptors. Transfer
large (128 kb) byte streams.

Extend the commandhelper.c with support for --readfd <fd> command line
parameter and convert the data receive loop to use poll and receive data
on multiple file descriptors (up to 3) and read data into distinct buffers
that we grow while adding more (string) data.
Signed-off-by: NStefan Berger <stefanb@linux.ibm.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>