Validate -l and -L arguments in named-checkzone

Validate -l and -L arguments in named-checkzone

The -l (max TTL) and -L (source serial) flags parsed their arguments
with strtol() and assigned the result directly to uint32_t with no
range check. A negative value such as -1 became UINT32_MAX, which made
-l silently disable the TTL cap it claimed to enforce, and out-of-range
values truncated to 32 bits without warning.

Switch both flags to isc_parse_uint32(), which rejects leading non-
alphanumeric input (catching '-'), checks ERANGE, and validates the
32-bit range, so an invalid argument now exits with an error instead
of being silently coerced.

Assisted-by: Claude:claude-opus-4-7