Provide C++ package "os" with File, Pipe, etc similarly to what is
provided on Go side. The package works through IO methods provided by
runtimes.

We need IO facility because os/signal package will need to use
pipe in cooperative IO mode in its receiving-loop goroutine.

os.h and os.cpp are based on drafts from wendelin.core:

https://lab.nexedi.com/nexedi/wendelin.core/blob/wendelin.core-2.0.alpha1-18-g38dde766/wcfs/client/wcfs_misc.h
https://lab.nexedi.com/nexedi/wendelin.core/blob/wendelin.core-2.0.alpha1-18-g38dde766/wcfs/client/wcfs_misc.cpp

/reviewed-on !17