mqtt2log - logging mqtt messages to a log file
         in a format suitable for ticktockdb.

The program subscribes to mqtt topics and logs these to a log file.
The log file is closed and a new opened after a specified number of
lines in the log file.

The logfile is named according to the time it is created at.

The program does chdir to /var/local/mqtt2log at start.  This is where the log
files will end up.

The program takes a configuration file as input.

The generic format of the configuration file is
	<white-space>* <tag> [<white-space> <arg>]* <end-of-line>
where the final argument continues to the end of the line.
Comment lines are ignored and have the format
	<white-space>* '#' <anything> <end-of-line>


The specific tags used by 'mqtt2log' are (<white-space> between <tag> and <args>
is implicit):
'mqtt' <ip-address> <port> <mqtt-id>
	<ip-address>	address of the mqtt broker, see mosquitto_connect().
	<port>		port of the mqtt broker, see mosquitto_connect().
	<mqtt-id>	id string of the client, see mosquitto_new().

'logfile' <max-lines>
	<max-lines>	When the program has written this amount of lines to
			a logfile, it is closed and a new is created.

'subscribe' <topic-selection>
	<topic-selection> topic(s) to subscribe to, see mosquitto_subscribe().
	Up to 32 selections can be subscribed to. '*' is replace with '#' as the
	latter may be used ad end-of-line comments later.  I.E. use '*' in place
	of '#' in the config file.

'topic' <topic> <metric> <tags>
	<topic>		An mqtt topic.
	<metric>	"metric" the topic name is translated to.
	<tags>		"tags" (to the end of line> used.
	When the topic arrives, it is written to the log file with the
	translation:
		'put' <metric> <time-stamp> <payload> <tags>
	See ticktockdb for further information on metric, time-stamp and tags.
	If the metric is '@ignore', no output is produced.  The <tags> argument
	must be present in the config file, but is ignored.
	If the <topic> is not recognised, it is written to the log file as:
		'bad' <topic> <time-stamp> <payload>

Example config file:
	mqtt 127.0.0.1 1883 test-id
	logfile 20000
	subscribe *
	topic temp/basement T loc=basement
	topic temp/kitchen  T loc=kitchen
	topic temp/test     @ignore anything




Shorthand commands to see status of mqtt2log: 

systemctl status mqtt2log
sudo systemctl start mqtt2log
sudo systemctl stop mqtt2log
sudo journalctl -t mqtt2log

sudo systemctl stop mqtt2log
sudo make install
sudo systemctl start mqtt2log
systemctl status mqtt2log

journalctl -f -t mqtt2log