The _types module can house any common type defintions used throughout
the rest of typeshed to keep defintions in sync.
First candidate is file descriptors where anything with `fileno()`
method is accepted. There were several different implementations in
various files that can be unified.
This patch:
* Adds some constants that although undocumented are exposed by the
module
* Modifies the signature of fcntl() to match reality
* Adds missing function stubs
* Modifies few existing constant declarations to match the repository
conventions
I believe the stub is complete now (and if it's not there isn't much
missing).
