Use `chkDots()` in more methods

[Bisaloo]

It is common that some arguments will be silently ignored due to the presence of ... in the argument list.

R Core have release the chkDots() utility almost 10 years ago so it can be warn when arguments are passed to ... but ignored: r-devel/r-svn@97d6701

But a number of base methods are not using this utility when they could. The goal of this issue is to list all the potential candidates before potentially opening a bugzilla issue / submitting a patch:

• toString.default()
• as.data.frame.data.frame()
• as.data.frame.vector()
• as.data.frame.matrix()
• cut.default()
• mean.default()
• ...

Other mechanism to guard against the presence of ...:

• split.default()