meson: allow building .standalone on demand

meson: allow building .standalone on demand

We can always build the standalone version whenever we build the normal version
(the dependencies are the same). In most builds standalone binaries would be
disabled. But it is occasionally useful to have them for testing, so move the
conditional to install:, so the binaries can be build by giving the explicit
target name.

The default of 'build_by_default' for executable() is sadly true (since meson
0.38.0), so need to specify build_by_default: too.

Also add systemd-shutdown.standalone to public_programs for additional testing.