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 <snorcht@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>