* lisp/mail/ietf-drums-date.el: parse-time-string replacement which is
compatible but can be made stricter if desired.
* test/lisp/mail/ietf-drums-date-tests.el (added): Add tests for
ietf-drums-parse-date-string.
* lisp/mail/ietf-drums.el (ietf-drums-parse-date): Use
ietf-drums-parse-date-string.