Make beacon options configurable on the command line

[larsks]

This PR adds the following options to axdigi2018:

• --enable-beacon, -b -- enable a periodic beacon every --beacon-interval seconds
• --beacon-text, -t text -- set beacon text
• --beacon-dest, -d callsign -- set beacon destination
• --beacon-interval, -i seconds -- set beacon interval (default 300)
• --beacon-path, -p digi1,digi2,... -- set beacon path

I have also run the code through clangd-format for consistent formatting, and added github workflows to check formatting and confirm the code builds for each pull request and push to master.