Resolves bootstrapping deadlock where downloading sslocal requires internet
access via the very proxy being deployed. Also handles distros (Arch) where
shadowsocks-rust is installed via package manager to /usr/bin instead of
/usr/local/bin.
Priority:
1. /usr/local/bin/sslocal exists → skip download
2. sslocal found in PATH elsewhere → symlink to /usr/local/bin/sslocal
3. not found → download from GitHub releases
Same logic applied to ssserver-rust in server/deploy.sh.
Also: stop conflicting shadowsocks.service before starting shadowsocks-client,
and detect pacman vs apt for privoxy install.