prevent imap directories being created with a dot

[Stephan Krebernik]

this leads to strange behaviors because the structure in data organizes imap directories with dots as well.

example:

creating an imap directory "bar.test" in your mail client will create

"poste/data/domains/foo.com/user/Maildir/.bar.test"

which equals "/bar/test" as imap folder.

[SH]

Duplicate of #146.