Help with $PATH masking troubles

Very often users have cabal at /usr/bin and then they install a new one and are confused as to why the version apparently has not changed. We do warn if the installed exe is not on the $PATH at all (iirc) but we don't warn about masking. We should.

Also, in the specific case of cabal-install perhaps we should consider some trick to re-exec the right version. That is, if the user runs /usr/bin/cabal then it could simply re-exec ~/bin/cabal, possibly with a $PATH warning. This would not be too hard to do (though of course it relies on the system version not being too old), but we'd want to work out when it's right to do it, vs when the user really did mean to run the system version.