This patch removes the SIGNAL service from core and moves its
functionality to the PD session. Furthermore, it unifies the PD service
implementation and terminology across the various base platforms.
Issue #1841
'block_for_signal' and 'pending_signal' now set pending flag in signal context
in order to determine pending signal. The context list is also used by the
'Signal_receiver' during destruction.
Fixes#1738
This patch changes the top-level directory layout as a preparatory
step for improving the tools for managing 3rd-party source codes.
The rationale is described in the issue referenced below.
Issue #1082
