bugfix: ompipe did emit many suspension messages for /dev/xconsole

(hopefully now) closes: https://github.com/rsyslog/rsyslog/issues/35
When it was present, but nobody reading from it. The problem
is the way the rsyslog v7 engine tries to resolve failures in outputs.
It does some retries, and along those lines some state information gets
lost and it is close to impossible to retain it. However, the actual
root problem is that ompipe does not reliably detect if it is able to
recover. The problem here is that it actually does not know this
before it does an actual write. These two things together mess up the
logic that suppresses invalid resumption/suspension messages
(actually, the plugin switches state really that often).
Nevertheless, the prime problem with /dev/xconsole (and probably
most other pipes as well) is that it gets full. So I have now added
code that checks, during resume processing, if the pipe is writable.
If it is not, resume is deferred. That should address the case.