posix-timer: Restore and fix one timer
posix-timer: Restore and fix one timer

This also add function to check if time represented by two numbers is
valid. I.e. for timespec(sec, nsec), sec and nsec must be > 0 and nsec
must be less when NSEC_PER_SEC.

Signed-off-by: Pavel Tikhomirov <2566b75157ba49177214ae1d5f51030018b1e134@gmail.com>
Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@parallels.com>

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