MINOR: proxy: permit to report version info for option deprecation

MINOR: proxy: permit to report version info for option deprecation

It's already possible to report that some options are not supported due
to build options by passing 0 instead of PR_CAP_* in the option's cap
field. Let's extend that by passing a non-zero value in the val field,
where the 3rd byte will be the major version and the 4th one the minor.
In this case haproxy will now indicate that support for that option was
removed in that version.