imap.el (imap-kerberos4-open): Work with recent `imtest's.

imap.el (imap-kerberos4-open): Work with recent `imtest's.
nnimap.el (nnimap-request-article): Use BODY.PEEK[] instead of
RFC822.PEEK if server support IMAP4rev1.
(nnimap-request-body): Use BODY.PEEK[TEXT] instead of
RFC822.TEXT.PEEK if server support IMAP4rev1.
(nnimap-request-head): Use BODY.PEEK[HEADER] instead of
RFC822.HEADER if server support IMAP4rev1.
(nnimap-request-article-part): Support bodydetail in response
data.