MINOR: log: explicitly ignore "log-steps" on backends

MINOR: log: explicitly ignore "log-steps" on backends

"log-steps" was already ignored if directly defined in a backend section,
however, when defined in a defaults section it was inherited to all
proxies no matter their capability (ie: including backends).

As configurations often contain more backends than frontends, this would
result in wasted memory given that the log-steps setting is only
considered on frontends.

Let's fix that by preventing the inheritance from defaults section to
anything else than frontends. Also adjust the documentation to mention
that the setting in not relevant for backends.