A true "default" config doesn't make sense for river. Everyone who uses
river seriously will customize their init script. Futhermore, the
current behavior of embedding the install path of the default system
config in the river binary is complex and prone to breaking.
Run the init command in a new process group and send SIGTERM to the
entire group on exit. Without doing this, only the sh invocation used
for the `sh -c` would receive SIGTERM.
This is particularly useful when starting a per-session server manager
as the init command.
