Better message when new cabal-install available

A common mistake new users of cabal-install seem to make is to run
`cabal update`, see the message "Note: there is a new version of
cabal-install available.", accordingly install a new version of
cabal-install by doing `cabal install cabal-install`, neglect to make
sure that the newly installed cabal-install executable is in their path,
and continue to run the cabal-install executable they had before.  The
next time they do `cabal update`, they see the same message and become
confused.

This commit changes the message in question to be more informative and
helpful.  In particular, if the currently running version of
cabal-install is older than the newest version available from hackage,
the user will not be told simply that "there is a new version of
cabal-install available", but rather will be informed exactly what new
versions are available, and what version they are currently running,
along with a hint to change their $PATH variable if it turns out they're
not running the version of cabal-install they thought they were.
s/$PATH variable/PATH environment variable/
We don't want to be Unix-specific.