modbusacap.c: Handle stateful Threshold events too

[joakimr-axis]

Describe your changes

These events have the topic2 string set as "Device1ScenarioXThreshold", where X = 1, 2 ... N. This patch also move the
ax_event_handler_unsubscribe calls prior to exiting the main loop, where they need to be.

Checklist before requesting a review

• I have performed a self-review of my own code
• I have verified that the code builds perfectly fine on my local system
• I have added tests that prove my fix is effective or that my feature works
• I have commented my code, particularly in hard-to-understand areas
• I have verified that my code follows the style already available in the repository
• I have made corresponding changes to the documentation