earlyoom.service: drop root privileges
earlyoom.service: drop root privileges

Run as a random unprivilege user instead of as root,
but add the capabilities CAP_KILL CAP_IPC_LOCK.

Supersedes https://github.com/rfjakob/earlyoom/pull/158

rfjakob/earlyoom,rfjakob/earlyoom,rfjakob/earlyoom,rfjakob/earlyoom