Implemented optional custom login name for mailboxes.

Added needed incremental sql statements.
Option can be enabled in interface config, mail tab.

install/sql/incremental/upd_0010.sql

+-- database patch for separate login for mailbox users
+ALTER TABLE `mail_user` ADD `login` VARCHAR( 255 ) NOT NULL ;
+UPDATE `mail_user` SET `login` = `email` WHERE 1 ;
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