Sync with Nana-gnus 7.1.0.15.

Sync with Nana-gnus 7.1.0.15.

* gnus-vers.el (gnus-revision-number): Increment to 17.

* gnus-start.el (gnus-product-variable-touch): Support multiple arguments.

* gnus-spec.el (gnus-search-or-regist-spec): New utility macro.
(gnus-update-format-specifications): Support new data structure of
`gnus-format-specs-compiled'.
(gnus-update-format-specification-1): Likewise.
(gnus-update-format): Support new data structure of `gnus-format-specs'.
(gnus-format-specs-compiled): Fix doc string.
(gnus-format-specs): Modify data structure.