Enable sieve regex MTA option

Sieve language extension MTA options: (0-2)
The  MTA  option controls whether Sieve filters may use the  Sieve regex extension  (the " " match-type). A value of  disables the use of   in all Sieve scripts. The default value is , meaning that   may be used in any Sieve script. As of the 7 Update 2 release, a value of  allows   to be used in  system-level Sieves only.

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

See also:
 * Sieve regex extension
 * Sieve language extension MTA options