merge recipe changes to .status.el on update too

Previously, the recipe in .status.el was only updated on init, since
only some properties are safe for change on init the unsafe ones were
never updated.  Therefore, we classify addtional properties as safe fo
change on update, and update .status.el after el-get-update too.

* el-get-core.el (el-get-as-string): Handle number argument.
* el-get-status.el (el-get-status-init-whitelist): Rename from
  el-get-status-recipe-update-whitelist.
(el-get-status-update-whitelist): New constant.
(el-get-classify-new-properties): Add OPERATION argument to choose white
whitelist to use.  The :builtin property is safe to change if it doesn't
change the :type.
(el-get-diagnosis-properties, el-get-merge-properties-into-status): Add
OPERATION argument.
* el-get.el (el-get-do-init): Call el-get-merge-properties-into-status
  with OPERATION 'init.
(el-get-post-update-build): Call el-get-merge-properties-into-status
  with OPERATION 'update.