More parameters for IMAP

- mail_max_userip_connections: set to default of 10
- imap_idle_notify_interval: set to default of '2 mins'

Using dovecotcfmulti since there are several parameters

Bugfix: mail_plugins should be set inside the 'protocol imap' block

There is a global mail_plugins that is already set in `dovecot::mail`.

Tested, correctly sets the values on my server.