Promote gvm-init.sh to be a first class function

I'd like to be able to do this at any time:

    $ GVM_SERVICE=foo.bar.com gvm init

Seems pretty obvious, so that's what the patch does.  The only thing I wasn't very clear on is why gvm-init.sh was in a bin dir with the executable bit set in the first place.  If there was a reason for that which I didn't understand I might have to revise the patch.