[MAJOR] distinguish between frontend, backend, ruleset and listen

[MAJOR] distinguish between frontend, backend, ruleset and listen

The notion of capabilities has been added to the proxy so that we
know whether a proxy supports frontend, backend, or rulesets. Given
this, some parameters are optionnal, some are ignored with a warning
and others are forbidden. It is now possible to write valid two level
configs without binding to dummy address/ports.