main: exit with code 0 on shutdown
main: exit with code 0 on shutdown

To avoid confusing systemd.

Fixes #823.

Message-Id: <316924e313934d944d4944ac15f6097739650c37@scylladb.com>

kjniemi/scylla,duarten/scylla,scylladb/scylla,duarten/scylla,raphaelsc/scylla,kjniemi/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,kjniemi/scylla,raphaelsc/scylla,raphaelsc/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla