disable SSLv3 in dovecot now that it is known to be insecure (POODLE)

SSLv3 is already disabled in Postfix (45e93f7d) and Nginx (51dd2ed7).

CHANGELOG.md

@@ -7,6 +7,7 @@ To-be-released
 Breaking changes:
 
 * On-disk backups are now retained for a minimum of 3 days instead of 14. Beyond that the user is responsible for making off-site copies.
+* IMAP no longer supports the legacy SSLv3 protocol. SSLv3 is now known to be insecure. I don't believe any modern devices will be affected by this. HTTPS and SMTP submission already had SSLv3 disabled.
 
 Control panel:
 

setup/mail-dovecot.sh

@@ -43,12 +43,13 @@ tools/editconf.py /etc/dovecot/conf.d/10-auth.conf \
 	disable_plaintext_auth=yes \
 	"auth_mechanisms=plain login"
 
-# Enable SSL, specify the location of the SSL certificate and private key files,
-# and allow only good ciphers per http://baldric.net/2013/12/07/tls-ciphers-in-postfix-and-dovecot/.
+# Enable SSL, specify the location of the SSL certificate and private key files.
+# Disable obsolete SSL protocols and allow only good ciphers per http://baldric.net/2013/12/07/tls-ciphers-in-postfix-and-dovecot/.
 tools/editconf.py /etc/dovecot/conf.d/10-ssl.conf \
 	ssl=required \
 	"ssl_cert=<$STORAGE_ROOT/ssl/ssl_certificate.pem" \
 	"ssl_key=<$STORAGE_ROOT/ssl/ssl_private_key.pem" \
+	"ssl_protocols=!SSLv3 !SSLv2" \
 	"ssl_cipher_list=TLSv1+HIGH !SSLv2 !RC4 !aNULL !eNULL !3DES @STRENGTH"
 
 # Disable in-the-clear IMAP and POP because we're paranoid (we haven't even