Memcached bugs (31404, 31406)

These changes modify the session and cache memcache(d) configuration and conf values usage in the following ways:

1. Separate conf variables introduced for memcache(d) host and port for cache and session, since it is possible to use different memcache servers (one for cache, another for session).

2. Separate conf variables for memcache and memcached, since if both extensions are present on the current php installation, it is possible to use different conf values (not really a very probable real world scenario, but still possible).

3. Related with the previous changes, the 'memcache' fieldset has been eliminated (no longer appended to the 'cache' form) and the corresponding memcache(d) fields have been added to the 'session' and 'cache' fieldsets.

4. Server conf array usage problems fixed in  libraries/joomla/session/storage/memcache.php and  libraries/joomla/session/storage/memcached.php in order to ensure proper construction of the storage objects and correct usage of the configuration values.