Use to ensure assembled IMAP commands fit in the resultant buffer to
be imap_exec()'ed.

RFC2683 suggested a limit of 1000, but asked servers to accept up to
8000.  Furthermore, RFC7162 bumped the client limit suggestion up to
8192.  So I don't believe any issues will be caused by this.

Most of these are increases are just to remove theoretical truncation
warned about by the compiler; I don't believe we've had reports of
actual command truncation due to a mailbox 1024 characters long.