MINOR: sample: add the "when" converter to condition some expressions

MINOR: sample: add the "when" converter to condition some expressions

Sometimes it would be desirable to include some debugging output only
under certain conditions, but the end of the transfer is too late to
apply some rules.

Here we take the approach of making a converter ("when") that takes a
condition among an arbitrary list, and decides whether or not to let
the input sample pass through or not based on the condition. This
allows for example to log debugging information only when an error
was encountered during the processing (sort of an extension of
dontlog-normal). The conditions are quite limited (stopping, error,
normal, toapplet, forwarded, processed) and can be negated. The
converter can also be chained to use more complex conditions.

A suggested example will be:

    # log "dbg={-}" when fine, or "dbg={... debug info ...}" on error:
    log-format "$HAPROXY_HTTP_LOG_FMT dbg={%[bs.debug_str,when(!normal)]}"