Fix handling of continuing unclosed multiline comments

[amnn]

Quoting from the README:

Filling a paragraph in an unterminated multi-line comment (starting with /* but with no matching */), or inserting a newline to continue the comment causes Emacs to use /* as the fill prefix rather than the intended *. This can be fixed by adding the closing */.

So if you have a buffer with this content, and the cursor at |:

/**
 * This is a multi-line comment which hasn't finished yet|

Running M-x comment-indent-and-newline currently (in Emacs 28.1) produces:

/**
 * This is a multi-line comment which hasn't finished yet
/**|

Instead of

/**
 * This is a multi-line comment which hasn't finished yet
 * |

This task is about fixing this behaviour without requiring that someone closes the comment proactively.

NOTE: This may not be possible, this bug is apparent in other major modes (tested with rust-mode, cc-mode).