[Dovecot] Smart IMAP proxying with imapc storage

[Timo Sirainen]

imapc settings have moved away from plugin {} section and mail_location.
Now instead use:

mail_location = imapc:

imapc_host = imap.example.com
#imapc_port = 143 # default

#imapc_user = %u # default
imapc_password = secret

imapc_ssl = no # or imaps or starttls
imapc_ssl_ca_dir = /etc/ssl/certs

Note the imapc_password change also. If passdb/userdb returned
userdb_pass/pass previously, return now instead
userdb_imapc_password/imapc_password.