config: make smtputf8 configurable through the API

config: make smtputf8 configurable through the API

the flag is simply a boolean which is used to:
* add smtputf8_enable = no to postfix' main.cf if it is disabled
  (the default is to enable it, and not adding it unconditionally,
  should cause the fewest surprises for users with modified templates)
* decide if locally generated mail should be scanned for utf8 headers
  and addresses (to set the parameter to the MAIL command)

This should match postfix own implementation w.r.t. smtputf8 behavior.

Additionally, since quite a few users need to disable it because
their downstream servers do not support it (Zimbra, OpenXchange,
MS Exchange), this should make for a better user experience.

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>
Reviewed-by: Dominik Csapak <d.csapak@proxmox.com>
Tested-by:  Dominik Csapak <d.csapak@proxmox.com>