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are cons pairs (function . help-echo) or nil.
(proced-refine): Use them.
(proced-format-alist): Allow alternatives.
(proced-descend): New variable.
(proced-sort): New arg descend.
(proced-sort-interactive): Repeated calls toggle sort order.
(proced-format): Accomodate changes of proced-format-alist.
Undefined attributes are displayed as "?".
(proced-process-attributes): New optional arg pid-list.
Ignore processes with empty attribute list.