ftoastr: depend on snprintf, improve comments * lib/ftoastr.c: Also mention Loitsch's draft. * lib/ftoastr.h: Require WIDTH to be nonnegative. This isn't needed in the current implementation, but it might simplify speeding up the code later. * modules/ftoastr: Depend on snprintf; this improves portability. Suggested by Bruno Haible in the same email.