Problem: Various inconsistencies in test files.
Solution: Add modelines where they were missing. Use Check commands instead
of silently skipping over tests. Adjust indents and comments.
(Ken Takata, closes#6695)
Problem: :argadd in empty buffer changes the buffer name. (Pavol Juhas)
Solution: Don't re-use the current buffer when not going to edit the file.
(closes#3397) Do re-use the current buffer for :next.
Problem: :argedit does not reuse an empty unnamed buffer.
Solution: Add the BLN_CURBUF flag and fix all the side effects. (Christian
Brabandt, closes#2713)
