lib/timeutil: avoid implementation-defined behavior

The algorithm for converting broken-down civil time to seconds in the
POSIX epoch time scale would produce undefined behavior on a toolchain
that uses a 32-bit time_t in cases where the referenced time could not
be represented exactly.

However, there are use cases in Zephyr for civil time conversions
outside the 32-bit representable range of 1901-12-13T20:45:52Z through
2038-01-19T03:14:07Z inclusive.

Add new API that specifically returns a 64-bit signed seconds count, and
revise the existing API to detect out-of-range values and convert them
to a diagnosible error.

Closes #18465

Signed-off-by: Peter A. Bigot <pab@pabigot.com>