Provide support for Solaris

This PR makes a few minor changes to ensure that BATS is able to run correctly on Solaris (tested on Solaris 10)
I don't understand this. How come the value of `getconf PATH` is not already on PATH? On other systems it seems that it is. Also, is `getconf` available on all platforms? Might be a good idea to silence stderr here in case it isn't.