Fixes in prog-fill-reindent-defun

Fixes in prog-fill-reindent-defun

Ensure that we don't consider lines such as

    switch (foo) {
        case 2:  // If point on 'c' in 'case'
            return 2;

not to be considered for filling.

Also make sure we check for the active region, to replicate the normal
fill-paragraph behavior.

* lisp/progmodes/prog-mode.el (prog-fill-reindent-defun): Adjust
regex, and make sure fill-paragraph checks for active region.

(bug#60360, bug#60322)