Changes mostly from Mark Lord.

- fix bugs in probe-time error handling
- only complete qc if not NULL
- check port-level polling flags

Enforce access rules where appropriate.

If the compiler is smart enough, this may buy us an optimization or two
as a side effect.

from me: change from OSL+GPL to GPL (with approval)

from Mark: Gets rid of an unneeded control bit, slightly increasing
throughput.