Commit 5a1ee6eb added support for disabling use of start-stop-daemon
via a placeholder in the default launch script. Unfortunately, that
placeholder was subsequently broken in 81a47639.

This commit reinstates the placeholder and adds tests to verify that all
of the placeholders in the launch script can be replaced and that they
have the required default values. Furthermore, it also allows the use of
start-stop-daemon to be configured via USE_START_STOP_DAEMON in an
app’s .conf file. This allows the configuration to be changed after the
app has been built.

Closes gh-4985