bug#61730: 30.0.50; Compiler warnings for delq and delete

[Mattias Engdegård]

There is now a consolidated and improved ignored-return-value warning on 
master. However, it does not yet warn about delq and delete, which this bug 
originally was about, but keeps a list of other functions for which such a 
warning makes sense. The false-positive rate seems to be low enough.

The warning can be suppressed using `ignore`, or `with-suppressed-warnings`.

Once the issues indicated by the warnings remaining on master have been dealt 
with, other functions will be considered for addition, depending on how useful 
this would be.