'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