Exit jackdbus cleanly on SIGTERM

[iibaranov-IG]

Fixes #971.

jackdbus called jackctl_setup_signals(0), which blocks the termination signals for later consumption by jackctl_wait_signals(). Unlike jackd, however, jackdbus never calls the wait function, so SIGTERM remains blocked indefinitely.

This installs a small async-signal-safe handler that records the requested termination signal in a sig_atomic_t. The existing D-Bus dispatch loop checks that flag and performs the normal controller cleanup path. Its existing 200 ms dispatch timeout bounds the exit delay.

Validation:

• Reproduced the current behavior in a private D-Bus session: jackdbus remained alive more than 3.2 seconds after SIGTERM and required SIGKILL.
• With this change, the same process exited through its normal cleanup path in about 214 ms with status 0.
• Confirmed the log records Controller received signal 15 and Controller deactivated.
• Completed a full python3 ./waf configure --dbus --classic and python3 ./waf build -j2 build in a clean Debian environment.