bridge, ws: Use "done" message instead of "eof"

This is clearer and makes more sense for non-stream channels

Fixes #1590
Closes #1604
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
ws: Remove unused server variable in cockpit-session

Suggested by Coverity.
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
bridge: Check return value from fopen() in test

Suggested by Coverity.
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
ws: Don't unreference null fixture pointer in test

Suggested by Coverity.
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
bridge: Check return values from unlink and log message

This would make debugging a bit easier.

Reported by Coverity.
Closes #1606
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>