fix issue of un-used levels and use of p.max for factor constraints in plot.envfit

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/vegan/pkg/vegan@2342 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
update (for r2342)

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/vegan/pkg/vegan@2343 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Merge branch 'r-forge-svn-local'