BUG/MEDIUM: htx: check the HTX compatibility in dynamic use-backend rules

BUG/MEDIUM: htx: check the HTX compatibility in dynamic use-backend rules

I would have sworn it was done, probably we lost it during the refactoring.
If a frontend is in HTX and the backend not (and conersely), this is
normally detected at config parsing time unless the rule is dynamic. In
this case we must abort with an error 500. The logs will report "RR"
(resource issue while processing request) with the frontend and the
backend assigned, so that it's possible to figure what was attempted.

This must be backported to 1.9.