putting documentation strings before function prototypes
include stdint.h in time.h to be able to use UINT32_MAX. Add
postcondition for __tp->tv_sec (no overflow) in clock_gettime.
add __PO_HI_UINT32_MAX constant for using in specifications
add constraint on sec field of __po_hi_time_t (<= UINT32_MAX)
add po_hi_types.h to be able to use constants in specifications
specification of __po_hi_add_times. Use a strong precondition (no
overflow for sum of sec), maybe have to refine that with an error behavior.