Commit only build for arch-unix (and no tools either)
This simplifies a lot the calculations.

  - all times are now living in a single structure
  - all fields are signed and 64-bit long
  - the 2038 bug is over (we still had 32-bit seconds)
  - no overflow in math or unexplained "unsigned" appearing
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>