Add support for Dovecot mailbox settings

[peter-mauritius]

Candidate for settings are

• use_namespace
• use_omap (use map instead of xattr)

[peter-mauritius]

Added support plugin configuration. Example:

const char *settings_poolname = mail_user_plugin_getenv(mbox->storage->storage.user, "rbox_pool_name");

The Configuration can be found in eg. 90-plugin.conf:

plugin {
  rbox_pool_name = mail_storage
}