bug#21134: 24.5; Default value of imap-ssl-program

[Milan Zamazal]

The default value of imap-ssl-program is

  '("openssl s_client -quiet -ssl3 -connect %s:%p"
    "openssl s_client -quiet -ssl2 -connect %s:%p"
    "s_client -quiet -ssl3 -connect %s:%p"
    "s_client -quiet -ssl2 -connect %s:%p")

This doesn't work when SSL is disabled and only TLS is available on the
IMAP server, e.g. in current Debian.  Moreover, default IMAP setting
shouldn't enforce usage of deprecated protocols.

imap-ssl-program default value should be updated (maybe just omitting
the -ssl3 and -ssl2 options would be a good idea, but I'm not an
expert).