Replace uses of POSIX open/close

[eelstork]

Protobuf related i/o in Caffe relies on POSIX open/close. These functions are not cross-platform.
If performance concerns are not critical, the proposed solution (kindly suggested by @willyd and @keenbrowne) may be adequate.

Previously, ReadXXX/ReadXXXOrDie, WriteXXX/WriteXXXOrDie variants were available; fail-slow variants had no uses in higher level APIs, possibly unsafe uses in tests and examples.

[ajtulloch]

• What's the perf change for loading a nontrivial model (say, a hundred MB) with this diff?
• Having the non-CHECK'ing APIs are useful for cases where you're embedding Caffe in a larger process, and a failure to read a file doesn't need (and shouldn't) take down the whole process. I'd be in favor of not removing them.

[eelstork]

@ajtulloch a performance dip is expected according to IstreamInputStream docs; I don't mind running a couple of tests if interested parties perceive that this is important - might you contribute data?

Although I agree that error handling should be customisable, the current implementation falls short. For sake, have a look at ReadProtoFromTextFile. An assertive check is carried out regardless, with the returned flag only covering parsing correctness. Is this expected behaviour?

Raising standard or custom i/o exceptions may be appropriate.

[willyd]

+1 for this as it will help many of us maintaining Windows forks of Caffe.