:url is update safe for :type http*, builtin

* el-get-status.el (el-get-status-update-whitelist,
  el-get-status-init-whitelist): Remove.
  (el-get-status-may-change-on-init,
  el-get-status-may-change-on-update): New functions safety
  predicates. (el-get-status-prop-class-plist): New constant, map
  properties to safety predicates.
  (el-get-classify-new-properties): Use
  el-get-status-prop-class-plist instead of
  el-get-status-update-whitelist and el-get-status-init-whitelist.