+++ This bug was initially created as a clone of Bug #1212862

While reviewing shadowsocks-rust I noticed that our packaging provides systemd
services that let the various daemons runs with full root privileges.

The daemons don't actually need root privileges. At most they need the
capability CAP_NET_BIND_SERVICE to listen on ports lower 1024. The upstream
repository contains example systemd services beneath the debian/ directory
that seem to be smarter than what we have:

shadowsocks-rust.service: this lets the service run as nobody:nogroup

shadowsocks-rust-server@.service: this allows running multiple configurations
of the service. The privilege drop logic is unclear though, the service still
runs as root, but with a capability bounding set of CAP_NET_BIND_SERVICE. This
is a bit of a strange setup though, a root user that has lost most of its
capabilities. I am unsure whether this is sufficient isolation.

For addressing this also bug 1216372 about the world readable configuration
directory and configuration files should be kept in mind. Introducing a
dedicated user and group for shadowsocks that has read access to these files
might be the cleanest approach.