Properly initialized signal sets.

Review: https://reviews.apache.org/r/17617
Added io::write (in C++11).

Review: https://reviews.apache.org/r/17618
Refactored io::write for C++03.

Review: https://reviews.apache.org/r/17619
Added io::splice (C++11).

Review: https://reviews.apache.org/r/17620
Refactored io::splice for C++03.

Review: https://reviews.apache.org/r/17621