Add SIGUSR2 handler in bridge plugin to re-load state.

[SanderVocke]

Carla's bridge plugin (via carla-single) is the best single plugin host I have found for Linux.

The only feature I am missing is being able to re-load the plugin state at runtime. That would enable reverting to a state saved earlier by sending a SIGUSR1. In my case I want to use that in a scripting project where I manage some running plugins, but also for manual usage it would be useful.

This pull requests adds a SIGUSR2 handler, which will re-load the plugin state from the state file (just as it does on start-up).

I figured that although Carla apparently doesn't need this feature, the change is small and harmless and makes Carla just a tiny bit better. I hope you will consider pulling this in.

How I tested:

• Build and install on Arch system with pipewire + pipewire-JACK
• carla-single lv2 http://gareus.org/oss/lv2/b_synth
• Twiddle some knobs
• Send SIGUSR1 to the carla bridge process. Confirmed a state file is now there.
• Twiddle more knobs
• Send SIGUSR2 to the carla bridge process. Confirmed the knobs are back where they were when saved.
• Restarted the plugin in the same folder. Confirmed loading state at startup is still working (knobs are where they were when saved).