MINOR: log/sink: detect when log maxlen exceeds sink size

MINOR: log/sink: detect when log maxlen exceeds sink size

To prevent logs from being silently (and unexpectly droppped) at runtime,
we check that the maxlen parameter from the log directives are
strictly inferior to the targeted ring size.

      |global
      |     tune.bufsize 16384
      |     log tcp@127.0.0.1:514 len 32768
      |     log myring@127.0.0.1:514 len 32768
      |ring myring
      |     # no explicit size

On such configs, a diag warning will be reported.

This commit depends on:
 - "MINOR: sink: simplify post_sink_resolve function"
 - "MINOR: ring: add a function to compute max ring payload"