Allows user session handlers to create session IDs by adding an optional
7th argument to session_set_save_handler() and a create_sid() method
to SessionHandler.
* Amended existing tests to cater for new functionality.
* Implemented fixes and optimisations recommended by NikiC
* Added create_sid to the registered interface. This was breaking
tests. It also now breaks BC for people implementing the interface
directly instead of extending the class.
A lot of code already existed to allow a custom create_sid handler, but
lacked a specific implementation.
Therefore I have added a 7th (optional) argument
session_set_save_handler, to allow a user function to be supplied for
session id generation.
If a create_sid function is not supplied, the default function is
called in its absence to preserve backwards compatibility.
Likewise create_sid only added to SessionHandler class, and not the
interface to maintain backwards compatibility. If the result is not
overridden, the default is called.
