Update overflow-checks option #3624

This is a follow up from the discussion in #3620.

Requested feature: Kani's options to enable/disable overflow:

$ kani --help | grep overflow
      --overflow-checks
          Turn on default overflow checks
      --no-overflow-checks
          Turn off default overflow checks

give the impression that they're relevant to all types even though they only control overflow for floating-point types:

We need to either change the option name to float-overflow-checks or just float-checks (since it also controls CBMC's --nan-check) or at least update the help to clarify that the option only applies to floating-point checks.