Add alpn-protos command line option

[ThijsFeryn]

I'd like to have a --alpn-protos command line option that allows me to define which application layer protocols I want to support.

From a Docker point of view, this would be a convenience, and would allow me to tackle the following issue on-the-fly:

alpn-protos is specified with more than one protocol while proxy-v2 is  not selected. This is a configuration error.

[daghf]

Turns out we already have this, it was just missing from the --help output.

Fixed via bd7806e