Improve progressive summaries in Rmail

Improve progressive summaries in Rmail

* lisp/mail/rmailsum.el (rmail-summary-progressively-narrow):
Renamed from 'rmail-summary-apply-filters-consecutively'.  Doc
fix.
(rmail-summary-invert): Renamed from 'rmail-summary-negate'.  Doc
fix.
(rmail-summary-currently-displayed-msgs): Doc fix.
(rmail-summary--exists-1): New function.
(rmail-summary-by-labels, rmail-summary-by-recipients)
(rmail-summary-by-regexp, rmail-summary-by-topic)
(rmail-summary-by-senders): Call 'rmail-summary-exists' to verify
that the summary is usable.