Calculate gnus-version correctly on Cygwin

* gnus.el (gnus-continuum-version): Do main calculations in integers.
  (gnus-continuum-version-1): New function, return a string.
* gnus-msg.el (gnus-extended-version, gnus-bug):
  Use gnus-continuum-version-1 instead of gnus-continuum-version.
Fix previous commit

* gnus.el (gnus-continuum-version):
* gnus-msg.el (gnus-extended-version): Simplify.

* gnus.el (gnus-continuum-version-1): Remove.
* gnus-msg.el (gnus-bug): Revert.
gnus-art.el (gnus-button-url-regexp): Make it match url in which punctuation characters follow parentheses (bug#14950)