(gnus-product-directory): New user option.

(gnus-product-directory): New user option.
(gnus-clear-quick-file-variables): New function.
(gnus-clear-system): Use `gnus-clear-quick-file-variables'.
(gnus-read-newsrc-file): Likewise.
(gnus-read-newsrc-el-file): Read product's variable files.
(gnus-product-read-variable-file-1): New function.
(gnus-save-newsrc-file): Save product's variable files.
(gnus-product-variable-touch): New function.
(gnus-product-variables-dirty-p): Ditto.
(gnus-product-save-variable-file): Ditto.
(gnus-product-save-variable-file-1): Ditto.
(gnus-product-quick-file-format): Ditto.