LetsEncrypt support

[greboid]

Currently there's no sensible way to use LE with DFBnc, there's support for PEMs, but you still have to restart DFBnc for it to notice the change in certificate.

It'd be nice if it supported generating and renewing certs on its own, but this seems a little out of scope as well as potentially more work than it should do.

The easiest solution would be to add support for it either noticing the files have change and reload the certs, or to listen for a signal and after receiving reload the certs.