Implement hermit-unlink

hermit-unlink now deletes all symbolic links that connect to $HERMIT_PROFILE.. i think.
I think the trailing slash on $HERMIT_PROFILE will be problematic.  For instance, this is my result for `find test` and `find test/` on the hermit test directory (snipped).

    $ find test
    test
    test/.environment
    test/.gitkeep
    test/.install-environment
    test/fetch
    test/fetch/.do-fetch

    $ find test/
    test/
    test//.environment
    test//.gitkeep
    test//.install-environment
    test//fetch
    test//fetch/.do-fetch

Notice that the paths returned by `find` in the second set of results are actually totally incorrect.

Also, `HERMIT_PROFILE` should be what `HERMIT_PROFILE_PATH` is in `hermit-add` for instance.