The previous code was overlooking fallback when neither symlink not exec was
The number of "variants" is getting too high, so I am consolidating this in a
single closure that should be easier to maintains.
This also ensure that fallback flags are always taken into account.
(they are not user code yet, but small experimentation shown that the feature
was working as intended.)
A a small side effect we need to check for symlink support more lazily and this
show up in the test in a couple of places.