Arch Linux support

As you will see, I've handled things differently for Arch. The main script is much smaller as some duties are handed to pacman and systemd:
- Port/IP/cache: systemd - caveat: check failures manually
- Installation: pacman - caveat: users can break it with the --nodeps flag
- chattr/resolvconf: pacman post/pre install script

Some additional changes to the systemd configs:
- Port 2053 (soltysiak, opennic JP) didn't work, so I've switched to 53
- Added PartOf= to backup so that restarting the main service restarts both

Known issues:
- The backup service will complain if it finds no secondary IP (best solution is add the missing addresses).
- Daemon-reload loads at installation the new (user) services. Existing Dnscrypt instances still run, but may fail if restarted without the new matching configs. Hopefully "curl &&" is enough.

Conclusion: While Arch may be simple, systemd is absolutely not.