With the introduction of VRE_sub() many VRE_exec() use cases went away:

- VRE_NOTEMPTY flag
- start offset
- ovector for capture groups

The subject string length is now optional and zero can be passed to signal
that the subject string is null-terminated.

There are no options for VRE_match() but an option argument is kept in
case we start exposing some in the future.