Folder subscriptions are handled by the IMAP folder.  All clients see
the same subscription list.

You subscribe only to message folders, not nesting folders.

Linux imapd restrictions on nesting folders:
    1 level deep only.
    Either a message folder or a nesting folder, not both.

Linux imaps keeps track of a users' subscriptions in file
$HOME/.mailboxlist on the server.  The file is opened and
closed for each imap conn, so you can edit it during an
imap session (which does not have a persistent imap connection).