Hello,

I found some oversights in mailcap-add-mailcap-entry for which I am
proposing three different solutions in the form of attached patches.
The later patches actually supersede the earlier ones, but I am leaving
them in for the case that my understanding of the functionʼs operation
is wrong.  I have to admit, though, that the mail has gotten somewhat
long for such a relatively minor matter, and I am unsure about the
format and formatting, so let me know if there are any issues with it.

mailcap-add-mailcap-entry calls at its end setcdr twice, one call nested
into the other:

  #+begin_src emacs-lisp
  (setcdr old-major
          (setcdr old-major
                  (cons (cons minor info) (cdr old-major))))
  #+end_src

Given that setcdr returns its second argument, i.e. the NEWCDR, I
believe that setcdr here is essentially called twice with the same
arguments.  Therefore, one of the setcdr calls is superfluous and should
be removed.