Closes #5442. Stricter checks and warnings during coercion of columns during DT[, LHS := RHS.

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/datatable@1277 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Closes #5357. DT[, LHS := RHS] (or its equivalent 'set') now provides warning instead of error when length(LHS)=0.

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/datatable@1278 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Minor fixes to .Rd and global variable to fully pass R CMD check.

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/datatable@1279 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Merge branch 'refs/heads/r-forge-svn-local'