package/dovecot: New package

[Thomas:
 - slightly adjust the prompt of options in the Config.in file
 - fix license: it's just LGPLv2.1, no exceptions. Parts of the code
   are under public domain, or under the MIT license, but the
   combination is under LGPLv2.1.
 - rewrap one comment in the .mk file.]

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>