Ensure Finit installs its files to /usr/lib/finit when building with the
merged-usr config option. For consistency we adjust --exec-prefix, which
is the base for $libdir, meaning also /libexec is moved to /usr/libexec.
Finit tracks the paths internally, we only need to update runparts probe.
Signed-off-by: Joachim Wiberg <troglobit@gmail.com>