fix #4410: Remove non-null host bits from CIDR when writing postfix config

fix #4410: Remove non-null host bits from CIDR when writing postfix config

This will drop non-null host bits from `mynetworks` CIDRs when writing
the `main.cf` postfix template.
Backwards-compatibility with old entries in `/etc/pmg/mynetworks` is
thus also preserved.

Add an additional comment to the mynetworks API, indicating that unused
fields can/should be dropped with the next PMG version.

No GUI changes. The entries are written to `/etc/pmg/mynetworks` as the
user enters them. Suggested by Stoiko, see discussion in v2 thread [0].

[0] https://lists.proxmox.com/pipermail/pmg-devel/2022-December/002247.html

Signed-off-by: Christoph Heiss <c.heiss@proxmox.com>