(proced-grammar-alist): Allow predicate nil. New attribute tree.

(proced-grammar-alist): Allow predicate nil.  New attribute tree.
(proced-format-alist): Use attribute tree.
(proced-tree-flag, proced-tree-indent): New variables.
(proced-children-alist): Renamed from proced-process-tree.  PPID
must refer to a process in process-alist.  Ignore PPIDs that equal
PID.  Children alist inherits sorting order from process-alist.
(proced-process-tree): New variable.  New function.
(proced-process-tree-internal, proced-toggle-tree)
(proced-tree, proced-tree-insert, proced-format-tree): New
functions.
(proced-mark-process-alist): Add docstring.
(proced-filter-parents): PPID must refer to a process in
process-alist.  Ignore PPIDs that equal PID.
(proced-sort): Throw error if attribute is not sortable.
(proced-sort-interactive): Restrict completion to sortable
attributes.
(proced-format): Include tree in standard attributes if
proced-tree-flag is non-nil.  Make header clickable only if
corresponding predicate is non-nil.
(proced-update): Use proced-tree.