Allow running as a symlink.

If the `percol` executable is a symlink, `abspath` returns the directory
containing the link, while `realpath` returns the directory containing
the target. So now I can create

    local/bin/percol -> local/src/percol/bin/percol

and I don't need local/src/percol/bin in my $PATH.
Merge pull request #49 from ChickenProp/master

Allow running as a symlink.