Extract cookie parameter construction from am_cookie_set().

We need to make sure that we use the same parameters when deleting
the cookie, so create a new function am_cookie_params() that
calculates the parameters.
Use proper cookie parameters when deleting the cookie.

To make sure that the cookie is deleted, we need to use the
same parameters (domain and path) as when we set the cookie.
Update the code to use the new am_cookie_params()-function to
set the parameters when deleting the cookie.
Include version 0.8.1 in the NEWS-file.

This release was branched from the 0.8.0-release, and was therefore
not included in the NEWS-file for the master-branch.
Update NEWS-file for version 0.10.0
Bump version to 0.10.0.