rbindlist segfault fix for factor columns + empty data.tables. Closes #5355.

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/datatable@1123 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Commit 1123 also fixed #5120. Added test.

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/datatable@1124 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Fixing rbindlist edge case segfault: rbindlist(list())

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/datatable@1125 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Added dsorted and csorted to forder.

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/datatable@1126 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Merge branch 'refs/heads/r-forge-git-svn-local'

Conflicts:
	pkg/inst/tests/tests.Rraw
Added 2 tests that forder doesn't pass yet.

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/datatable@1127 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
dsorted and csorted now used in forder's column loop as well,  passes checks and timings.

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/datatable@1128 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
forder=data.table:::forder added to tests.Rraw to pass R CMD check

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/datatable@1129 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
setkey switched to forder

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/datatable@1130 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Merge branch 'refs/heads/r-forge-git-svn-local'
Clearer explanation in NEWS for bug fix #5305