lib-sieve: vacation extension: Use Microsoft X-Auto-Response-Suppress header...

lib-sieve: vacation extension: Use Microsoft X-Auto-Response-Suppress header to prevent unwanted responses.

We've seen a few situations where Pigeonhole and Microsoft products start a useless machine conversation.
At least older versions of Outlook ignore the standards on multiple points to make this happen.
We prevent this now by recognizing and generating the X-Auto-Response-Suppress header.