linux: fall back to a free proxy port when the configured one is taken

[misantronic]

On Linux handhelds a taken proxy port (8080 by default) made the start fail with "Port 8080 already in use - change proxy_port in config.json", which gave users no way to continue short of finding and editing the file.

• When the configured port is busy and no healthy service is running, the start now takes the next free port (up to 50 above), saves it as proxy_port in config.json and logs a warning. If none is free, the old error is raised.
• The menu starts the service before patching the emulator configs, so the patched host:port matches the port that was actually bound.
• The old test for the error is replaced by tests for the fallback and for running out of ports.

Not covered: the DarkOS systemd path, which keeps its old behavior.

Tested

muOS device: stopped the service, held 8080 and 8081 with other listeners, then ran start-proxy. The log shows Port 8080 is in use, switched proxy_port to 8082, the proxy listened on 8082, and cheevos_custom_host was patched to 127.0.0.1:8082. The real install was restored afterwards.

[misantronic]

RAOfflineProxy-muOS-v2.2.0-alpha1-portfallback.muxapp.zip