cqueue constructor that returns errno

[daurnimator]

I currently have some code that does this:

	local cq do -- Allocate cqueue first up, as it can throw when out of files
		local ok, err = pcall(cqueues.new)
		if not ok then
			local errno = ce.EMFILE
			return nil, ce.strerror(errno), errno
		end
		cq = err
	end

==> I have to guess that the error was due to running out of file descriptors, but this is not always correct.

It would be nice if there was a (alternate?) cqueue constructor that would return the actual errno.