api/postfix: add 'decode-headers' to postfix queue read

api/postfix: add 'decode-headers' to postfix queue read

often, the users want to show the *decoded* header, iow. they want
to see the readable subject,from,to, etc. not the quoted-printable
versions.

so add a new parameter that decodes the header lines as we read them
using MIME::WordDecoder's 'mime_to_perl_string'.

for backwards compatibility, this is not the default in the api

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>