Revert "tail: make help text for -n +N syntax more correct"

This reverts commit 3305c008ed6084f58b59dde5198ae92e3a458e46.
Revert "head: support -n -NUM and -c -NUM"

This reverts commit 40c6da433f0900d451e46da86422d89f3878d40c.
Revert "head,tail: use common suffix struct. simplify help text."

This reverts commit af0255f49681bcc78830a56af885e891eca210f4.