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