Adjusting dovecot+postfix, discount, ca-certificates.

ca-certificates uses an 'update-ca-certificates' script that relies on 'c_rehash' provided by OpenSSL. If OpenSSL is not present, the update-ca-certificates script will display an error but not actually return an error code and so butch finishes installation as if nothing was wrong. This results in a wget that can't check certs later down the line.

discount was swapped to using github for a mirror when the original site experienced a prolonged outage. Unfortunately github now redirects to HTTPS only for downloads and is no longer an option. I've pointed the URL back to the author's server, which is now available again. 

dovecot+postfix have been updated to specify a SSL cipher policy where previously none was specified. postfix's recipe has been fixed so that the conf files in KEEP are actually installed rather than butch overwriting them during its final stage. dovecot by default includes a README pointing to a /share location with example confs, this update bringd postfix in line: its example confs have been moved to /share and a README explaining basic usage has been added.