greylist: make netmasks configurable

greylist: make netmasks configurable

Instead of hardcoding the netmask used for comparing greylistentries to
the current ip (24 for ipv4 and 64 for ipv6) - make them configurable in
pmg.conf

This should help with some cloud providers who send the same mail with
different ips from a large network - which all get greylisted separately.
In the worst case the sending cloud drops the mail, after it got defered
too often.

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>