We were returning a zero-length ByteBuffer in some cases. We should be
consistent about this and always return a null ByteBuffer if nothing
was read at all.
There's some confusion between the write syscall and CIODevice::write()
here. The internal write() returns a boolean, and has already whined
in case the syscall failed, so we don't need to do that again.
Add a trivial CSafeSyscall template that calls a callback until it stops
returning EINTR, and use it everywhere we use select() now.
Thanks to Andreas for the suggestion of using a template parameter for
the syscall function to invoke.
