A client may register a signal handler to be notified whenever the
RTC value was changed, i.e., a mis-configured clock was synchronized,
by calling 'set_sigh()'.
Issue #3450
Instead of returning an uint64_t value, return a structured time stamp.
This change is only visible to components using Rtc_session directly.
Fixes#1381.
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
