Added new sessioncookiedomain setting to session handling section.
* allows you to change the domain that the Moodle cookies are available
from. This is useful for Moodle customisations (i.e. Squirrelmail SSO
or enrolment plugins) that need to share Moodle session information
with a web application on another subdomain.
* Will NOT work if the moodle host does not have a domain - i.e. just a
hostname, e.g. 'localhost' or 'myhostname'. Needs a FQDN
* Currently the setting is set to PARAM_TEXT length 50 since PARAM_HOST
does not allow a leading dot e.g. '.mydomain.com'
* TODO: do we make up a new PARAM_COOKIEDOMAIN which is the same as
PARAM_HOST but allows leading dots? Using PARAM_HOST and prepending a
dot may not always be desirable.
