MINOR: log: support 'to' parameter fo do-log action

MINOR: log: support 'to' parameter fo do-log action

Similar to the "to" argument for "log" directive, the optional "to"
parameter allows to specify a list of comma separater loggers section
names to be used as log destination for the logs produce by the do-log
action.

However, while the "to" argument from "log" directive adds up loggers
to existing loggers of the proxy, the do-log "to" argument actually
replaces the loggers configured on the proxy! Indeed, the ultimate
goal is to be able to filter which loggers sections should receive
do-log logs, which by default are sent to all loggers from the proxy.