Split counter-intuitive coTURN option `-c` into two options

[debuglevel]

I'm always quite confused of -c because it does two quite different things, depending on whether -v is given:

• -c server:secret installs coTURN on teh current server
• -c server:secret -v focal-250 tells BBB to use a coTURN server (possibly on a different machine)

I'd suggest to split this into two options:

• --use-coturn server:secret
• --install-coturn server:secret