Enhance Maildir support

[tommairs]

The current maildir support is rudimentary and adequate for basic testing but should be made more robust and save mail files sorted by user name and potentially by domain. This would enable installing Dovecot or some other webmail interface for management.

Current docs are here: https://docs.kumomta.com/reference/kumo/make_queue_config/?h=maildir#example-of-using-the-maildir-protocol

Ideally, there should be options for where to locate the message store and set dir and file permissions:

basedir = "/var/tmp/maildir"
dirmode = 775
filemode = 664

There should also be options for creating new folders based on the localpart of the destination address and optionally to create dirs based on the domains as well. IE: /var/tmp/maildir/new/kumomta.com/tom/
create_users = true
create_domain_paths = true

FInally, there should be options to capture mail for all domains or only for select domains.
all_domains = false
domains = ( "example.com", "kumomta.com" )