Send data pending status as part of payload instead of an invalid error

Send data pending status as part of payload instead of an invalid error

This ensures that the session daemon's logs don't contain false
positives every time a data pending command is replied-to.

The use of invalid error codes 0 and 1 causes the log to contain
misleading entries of the form:

Sending response (size: 16, retcode: Unknown error code (0)) [...]

This commit also proves that it is possible, contrary to what the
previous comment indicated, to send a boolean value over a socket
without using undocumented error codes.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>