Use boolean values directly in rmailsum

Use boolean values directly in rmailsum

* lisp/mail/rmailsum.el (rmail-summary-invert):
(rmail-summary-direct-descendants):
(rmail-summary--walk-thread-message-recursively):
(rmail-summary-by-thread):
(rmail-summary-by-labels):
(rmail-summary-by-recipients):
(rmail-summary-by-regexp):
(rmail-summary-by-topic):
(rmail-summary-by-senders): Use the boolean values in
'rmail-summary-currently-displayed-msgs' and similar directly instead
of comparing them to t and nil.