Add a mode control for derp server, and add a "manual" mode
to get derp server certificate. Under manual mode, certificate
is searched in the directory given by "--cert-dir". Certificate
should in PEM format, and use "hostname.{key,crt}" as filename.
If no hostname is used, search by the hostname given for listen.
Fixes#2794
Signed-off-by: SilverBut <SilverBut@users.noreply.github.com>
