Make proced-update preserve refinements

Make proced-update preserve refinements

Make proced-update preserve refinements by creating a new buffer local
variable proced-refinements which stores information about the current
refinements and is used by proced-update to further refine
proced-process-alist in the case it is non-nil.  The result is that
refinements are not immediately cleared when a proced buffer is
updated with proced-auto-update-flag non-nil.  proced-revert
maintains its current behaviour of clearing any active refinements.

* lisp/proced.el (proced-refinements): New buffer local variable
which tracks the current refinements.
(proced-refine): Set 'proced-refinements' variable and defer setting of
'proced-process-alist' to 'proced-update'.
(proced-update): Take into account 'proced-refinements' when setting
'proced-process-alist'.
(proced-revert): Set 'proced-refinements' to nil prior to calling
'proced-update'.