Be less strict about parameters at startup

[Yves Rutschle]

          ok so:

• make illegal arguments a warning rather than a failure. maybe only sometimes, as some configuration might be just stupid (e.g. no --listen option at all).
• PID file: arguably we can also make that just a warning: it's used by "the environment" (the distribution) to keep track of sslh, and if "the environment" fails to let us write the PID file, it's "the environment" that also loses the ability to stop sslh automatically.

Probably this deserves to be new issues :-)

Originally posted by @yrutschle in #416 (comment)