symlink-bindir does not play nice with program-suffix

(Imported from [Trac #924](http://hackage.haskell.org/trac/hackage/ticket/924), reported by guest on 2012-03-05)

I'd expect symlinks to be created with the program-suffix. Instead what happens is that cabal-install correctly suffixes the installed binary, but attempts to create a symlink to it from an unsuffixed name, which can fail if the unsuffixed name already exists in the symlink-bindir.

~ $ cabal --version

cabal-install version 0.13.3

using version 1.14.0 of the Cabal library

Example:

gruff $ cabal install --program-suffix=-fixed

...[snip]...

Warning: could not create a symlink in /home/claude/opt/bin for gruff because

the file exists there already but is not managed by cabal. You can create a

symlink for this executable manually if you wish. The executable file has been

installed at /home/claude/.cabal/bin/gruff-fixed

(The symlink already existed in this particular case, because I wanted to install my development program with a suffix to avoid clobbering the known working version.)

Start by locating cabal-install's handling of --program-suffix and symlink-bindir, then reproduce the reported command with a suffixed executable and an existing unsuffixed symlink. Done means the created symlink uses the program suffix and does not incorrectly target or conflict with the unsuffixed name.

Written by the indexing model from the issue text.