allow for optional 'ipv6:' prefix in transports

allow for optional 'ipv6:' prefix in transports

according to the smtp(8) and transport(5) manuals literal ipv6 addresses must
be written as '[ipv6:2001:db8::ff]', in accordance with rfc2821 [0].

Postfix does work irrespective of this prefix (as it has been working until
now), but we should allow for administrators to enter the addresses with the
prefix present.

[0] http://www.postfix.org/IPV6_README.html

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>
Reviewed-By: Dominik Csapak <d.csapak@proxmox.com>