Simplify some loops and cons

Simplify some loops and cons

* lisp/net/mailcap.el: Replace cl with cl-lib.
(mailcap--get-user-mime-data, mailcap--set-user-mime-data):
(mailcap-parse-mailcaps, mailcap-parse-mailcap-extras):
(mailcap-possible-viewers): Use push and dolist where possible.
(mailcap-viewer-passes-test): Remove unused binding.
(mailcap-add-mailcap-entry): Use push.
(mailcap-mime-info): Remove unused binding.  Use push.
(mailcap-parse-mimetypes): Use dolist.