Buildkite agent stops abruptly when stopped with systemctl

See PR #1075
I assume someone meant to set this to infinity,
the current setting of 0 means 0 seconds, it does not mean disabled/infinity

Start by reviewing PR #1075 and the systemctl stop configuration it changes. Confirm how the current setting of 0 is interpreted, then verify that the setting represents an unlimited stop period and that the agent no longer stops abruptly when managed with systemctl.

Written by the indexing model from the issue text.