Enable sieve memcache MTA option

Sieve language extension MTA options: (0-2)
New in the 8.0 release. The  MTA option controls whether  Sieve filters may use the    operator  to access and manipulate stored data using the memcache protocol. A value of  disables the use of   in all Sieve scripts. The default value is , meaning that   may be used in any Sieve script. A value of  allows    to be used in system-level Sieves only.

When  is set to ,  then any attempt to use   will result in a Sieve error  "Memcache access has been disabled".

When  is set to , then  user-level attempts to use   will result in a Sieve error  "Memcache only allowed in system-level sieves".

See also:
 * Sieve filters
 * Sieve memcache extension
 * Memcache MTA options
 * Sieve language extension MTA options