[dbsystel] Bump version to v5.2.1+dbsystel2

[dbsystel] Add pom.xml

Usually, Gradle’s `maven-publish` plug-in is responsible for generating
a POM file dynamically at build time.

However, thanks to the excellent work of @MrDOS and others, upstream's
Gradle-6-based process is currently being replaced [1] by a
Gradle-8-based one. The latter uses a different, more convenient method
to generate the POM.

In DB Systel’s fork, we’re not going to adopt that Gradle-8-based
build process for production until upstream PR NeuronRobotics#238 is properly tested,
merged, and included in a stable upstream release.

Given that we need to get two critical bugfixes that we’ve cherry-picked
into production quickly, we’re going to accept some duplication and
commit the POM to Git for the time being.

[1]: NeuronRobotics#238

address NeuronRobotics#180

No exits from native code

Adding freebsd support for Device Removal event

Device Removed Event Added

I am kinda excited, this is the first new feature added to RXTX in 11
years! it also happens to fix a 15 year old bug, present in the library
since USB to serial adapters were invented. The original developers
never imagined the event of removing hardware, because back then Serial
ports were built into the processor and could never be removed or added.

This feature will now add a user level event when the hardware is
removed. The user can then perform a shutdown procedure, since a
connection will be needed after the hardware is removed.

This also causes the Monitor thread that used to become a zombie to die
properly.

testing for the change in Monitor thread

Adding freeBSD libraries

Adding FreeBSD binaries

Jumping to 5.0.0 to indicate latest version.