rulesystem: matchfield: match all headers not only the first

rulesystem: matchfield: match all headers not only the first

currently the match field uses $entity->head->get in scalar context,
which only returns the first matching header (see [0])

switch over to using get_all in list context and iterating over all
headers makes it possible to match subsequent headers.

while it is uncommon in general - the Received headers are usually not
restricted to one - reported in our community forum:
https://forum.proxmox.com/threads/.109629/

[0] https://metacpan.org/pod/MIME::Head#Getting-field-contents
Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>