fixes a crash on systems that dont have utf7 in iconv.

2002-08-28  Not Zed  <NotZed@Ximian.com>

        * providers/imap/camel-imap-utils.c (imap_mailbox_encode): Chagned
        to use camel_utf8_utf7 code.
        (imap_mailbox_decode): As above, using camel_utf8_utf7.  'UTF-7'
        isn't a widely support iconv() codeset, and besides the new code
        is simpler.

        * camel-utf8.[ch]: robust utilities for working with utf8 and utf7.