pstream: Allow reading/writing through srbchannel

pstream: Allow reading/writing through srbchannel

For writing, we prefer writing through the srbchannel if one is available,
and we have no ancil data to send.

For reading, we support reading from both in parallel. This meant replicating
a struct used for reading, so a lot of this patch is just a search/replace in
do_read to use the appropriate channel for reading.

Signed-off-by: David Henningsson <david.henningsson@canonical.com>