Refactor Awaiter with PselectArgument
Hold HandlerConfiguration in Awaiter

The Awaiter implementation is going to depend on HandlerConfiguration to
support signal handling in upcoming commits.
Pass signal mask to pselect
Use registerTrigger function to construct PendingEvent

The FileDescriptorTrigger variant type is introduced as the PendingEvent
now holds file descriptor triggers only.
Call signal handlers after pselect
Reorganize PendingEvent as a class
Keep signal handler cancelers in PendingEvent
Share PendingEvent to enable weak reference
PendingEvent can be fired more than once
Register signal handler to HandlerConfiguration
Add AwaiterSignalTest
Merge pull request #95 from magicant/feature/signalawaiter

Add signal handling in Awaiter