[9.18] fix: dev: Change the NS_PER_SEC (and friends) from enum to #define

New version of clang (19) has introduced a stricter checks when mixing
integer (and float types) with enums.  In this case, we used enum {}
as C17 doesn't have constexpr yet.  Change the time conversion constants
to be #defined constants because of RHEL 8 compiler doesn't consider
static const unsigned int to be constant.

Closes #4845

Backport of MR !9313

Merge branch 'backport-4845-change-NS_PER_SEC-type-from-enum-to-integer-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!9340