Fix PR#16029, predvars being dropped by drop.terms.

git-svn-id: https://svn.r-project.org/R/trunk@66944 00db46b3-68df-0310-9c12-caf00c1e9a41
Improve r66944 to handle logical indexing as well.

git-svn-id: https://svn.r-project.org/R/trunk@66945 00db46b3-68df-0310-9c12-caf00c1e9a41