Improve configuration dump to log

Refactor much of the new code in log.c

Add new functions: cf_load_strict() and cf_reload_strict() for use by the
server, which does not want to overwrite its in-memory config at all if there
is any defect in the config file.  However, commands always want to overwrite
the default in-memory config, even with a defective config file, so that
logging has a chance to succeed.

Added `log_format.dump_config` boolean config option