Fix: runas worker attempts to send invalid fd to master

Fix: runas worker attempts to send invalid fd to master

Commands which return a file descriptor (i.e. RUN_AS_OPEN) attempt
to send the resulting file descriptor even on failure. However,
this is not permitted by the UNIX socket interface.

As a result, skip the reception of the file descriptor payload
when a command fails. The 'master' end is also adapted to skip
the reception of the file descriptor in the case of an error.

A check has also been added to ensure that the 'master' end does
not attempt to send invalid file descriptors to the worker process.

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