Extract PselectApi abstract class from Api

The methods in PselectApi will be exposed to the Awaiter implementation
class. To protect the API methods from other part of the program that
depends on the Api class, the methods are extracted as the PselectApi
class.
Awaiter depends on PselectApi

This commit is a refactoring to reduce unnecessary dependency. The
Awaiter class implementation now depends on the PselectApi class instead
of the Api class.
Add HandlerConfigurationApiFake test helper class

The new class is a copy of the SignalNumberSetFake and SigactionApiFake
classes, which will be removed soon in favor of the new class.
Add TimeApiFake test helper class
Add HandlerConfigurationApiDummy test helper class
Add PselectApiStub test helper class
Use new test helpers for Awaiter tests

The new test doubles do not depend on irrelevant API classes.