In POSIX, improve the diagnostic for the "use $method" instead.

In POSIX, improve the diagnostic for the "use $method" instead.

In the error message, name the POSIX function that was called, as well as
the suggested replacement method. This rephrasing was in the patch
supplied by Aristotle Pagaltzis, but I have retained the existing POSIX
use of :: when describing the method, because given two less than great
choices, I'm inclined to favour retaining the status quo and one change
over two changes.