New / Upcoming features for kplex 1.5

[Keith Young]

kplex 1.5 is a way off beta testing yet but there's some new features people may be interesting and testing is always welcome.  If something is working the feedback that it does work is useful as well as feedback that it doesn't.

The "develop" branch on github has 3 significant new features:

* Internationlization of error messages.  Currently Dutch is the only other language which kplex application error messages have been translated to, but everyone should now get system error messages generated by kplex in a language appropriate to their locale.

* Application level heartbeats.  These are proprietary sentences which can be sent out of an interface at regular intervals.  Possible uses include letting ais aggregation sites know that your udp-sending station is still online when you have no data to send

* Enhanced checksumming options.  kplex can now add missing checksums to sentences before forwarding them on.

Details here:

http://www.stripydog.com/kplex/beta.html#features

Additionally the "xfilter" branch on github contains the above features plus what turned out to be a big chunk of work: "external filters".  This feature allows users to incorporate their own programs or scripts into kplex's input or output.  Many people have asked for this and the previous advice involved multiple copies of kplex or tricks with named pipes but this should simplify things.  In this branch, two new options, "xifilter=" (for input filters) and "xofilter=" (for output) lets you modify (drop/add/alter) input or output.  The argument to these options should be a program or script with the following properties:

* Reads from standard input

* Writes to standard output

* Manages standard error if needed

* Is line buffered / flushes output after writing a newline

* Can be terminated by SIGTERM

* If the program adds or modifies sentences and correct checksums are required by downstream applications, either ensures checksums on added/modified sentences are correct or ensures checksums are not present and leverages the new "checksum=" options "add" or "addonly".

If the program requires arguments, the entire command line should be quoted in the configuration file.

This change involved a lot of surgery to kplex so the more testing it gets the better.  Feedback and suggestions appreciated!