Logging revamp

Error levels

Level	Syslog	Meaning	Comment
CRITICAL	CRIT	Critical error, agent will stop	Some of OUTPUT_LEVEL_ERROR messages will be re-classified as CRITICAL
ERROR	ERR	Non-critical error, agent will continue	Bulk of OUTPUT_LEVEL_ERROR messages
WARNING	WARNING	Not an error, but user has to be notified	New level. Will contain e.g. deprecation warnings
INFO	INFO	"What does agent do?"	OUTPUT_LEVEL_INFORM, OUTPUT_LEVEL_CMDOUT, OUTPUT_LEVEL_LOG go there, plus promise implementations have to be adjusted to report more on this level
VERBOSE	DEBUG	"How does agent do it?"	Current OUTPUT_LEVEL_VERBOSE

Changes in language

1. syslog_host, syslog_port should redirect all syslog messages to specified syslog daemon
2. log_level in body action should mean "all messages up to this level", not "all messages with this level".
3. In case promise does not have log_level, all events up to WARNING should be reported.