Use LMTP only if necessary

Defaultvalues for virtual_transport switched back to maildrop like in 3.0.5.x. If dovecot is used, configure_dovcot switches to dovecot also like in 3.0.5.x. lmtp is only used in future if Uid-Mapping is enabled -> therefor it is necessary. Switching is implemented in server-postfix-plugin.
Validate Mail/Webuser-UID-Mapping: only enable, when Single-Server-Setup. Switching only possible unless a user was created.

Reason for this change: Few months ago a feature for mapping web-user-linux-uid to mail-user was added to sum up disk quota over webspace and mailusage. This feature required switching mail-delivery virtual_transport from maildrop or dovecot to lmtp. LMTP has one big disadvantage: X-Original-header will get lost when deliver a BCC-mail to a mailbox.
Additional this feature only works if mail and web-files lay on the same server. and if changing this value, directory-permissions would be necessary to change, so it will only be possible if there is NO mail user created yet.

See merge request !128

Use dovecot or maildrop in default-cases
Switching to LMTP and back included in server-postfix-plugin
validator added to disable switching uid-mapping when already created mailusers or in multiserver-environment

Outgoing email copy

added ability to have outgoing email cc'd to another email address

See merge request !127

fices installation of postfix map file

outgoingEmailCopy

Master

See merge request !124

Fix initial ip select

See merge request !122

Custom tranport fix

See merge request !121

Master

See merge request !120

Master

See merge request !119

Convert to reseller

Fixes FS#925 http://bugtracker.ispconfig.org/index.php?do=details&task_id=925

See merge request !117

Master

See merge request !118