If flags are used instead of positional arguments, also expose cert and key paths falling back to existing defaults.

This builds on top of @cespare's conversion from positional arguments to flags and doesn't make a lot of sense otherwise, so feel free to leave this on the cutting room floor.  Adding optional flags for the certificate and key paths makes it nicer to use pollen without having to put them in /etc.

I'd also second the vote for flags over positional arguments.  It's nice to be able to run pollen stand alone without any arguments and get the author-provided defaults.  What you get for free with -h is nice too:

$ pollen -h
Usage of pollen:
  -bytes=64: The size in bytes to transmit and receive each time
  -cert="/etc/pollen/cert.pem": The full path to cert.pem
  -device="/dev/urandom": The device to use for reading and writing random data
  -http-addr=":80": The HTTP address:port on which to listen
  -https-addr=":443": The HTTPS address:port on which to listen
  -key="/etc/pollen/key.pem": The full path to key.pem

I totally respect the decision if positional arguments are the way to go for your use case.