Extend ignored-return-value warning to more functions (bug#61730)

Extend ignored-return-value warning to more functions (bug#61730)

Warn when the return value of certain functions is unused.  Previously
this was only done for side-effect-free functions, and for `mapcar`.

These are functions where the return value is important for correct
usage or where ignoring it is likely to indicate a mistake.  The exact
set of functions is tentative and will be modified as we gain a better
understanding of which ones to include.

The current set comprises higher order functions such as `mapcar`
which are not primarily called for the effects of their function
arguments, and list-mutating functions like `nreverse` whose return
value is essential.

* lisp/emacs-lisp/bytecomp.el (byte-compile-form): Add list of
functions to warn about when their value is ignored.
* etc/NEWS: Announce.