ruledb: modfield: properly handle fields spanning multiple lines

ruledb: modfield: properly handle fields spanning multiple lines

this commit fixes aa3a005a5e7e3ad69ed35e9957fa514f73315c72

MIME::Words::encode_mimewords does not deal with multiline headers
(the warning about this being a 'quick and dirty' solution [0]
partially tells as much).

Instead - split the replacement value after variable substition on:
'\r?\n\s*' (to capture multi-line values like __SPAM_INFO__, but also
already folded headers, which are separated by '\r?\n\s+') and do the
substitution for each line seperately.

reported in our community forum:
https://forum.proxmox.com/threads/.118001/

[0] https://metacpan.org/pod/MIME::Words#PUBLIC-INTERFACE

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>