Add Nop class
Add SignalErrorCode enum class and SignalErrorCategory class
Add UnimplementedApi test helper class
Add SignalNumberSetFake test helper class
Add SignalMaskApiFake test helper class
Add SignalNumberSet::clone function
Add SigactionApiFake test helper class
Rename Api.cc to ApiImpl.cc

To separate object files into Api.o and ApiImpl.o. Only the latter
depends on capi.o.
Add sigprocmaskBlock & sigprocmaskUnblock functions to Api
Add HandlerConfiguration class