MINOR: log: support 'to' parameter for "log" directive

MINOR: log: support 'to' parameter for "log" directive

"to" is like "global", this is a special (optional) argument to be used
alone without ordinary log's arguments. It expects a comma separated list
of "loggers" section names which indicates which loggers section to inherit
loggers from. (each loggers section defines a number of "log" directive,
called logger).