[2.12+2.13] make abstractions/postfix-common compatible with latest postfix profiles

Even if we don't backport the latest postfix profiles (in extras) to 2.12 and 2.13, making the abstraction compatible with them (by adding peer=postfix-master rules in addition to the path-based ones) makes things much easier for people who want to use the latest profiles.

Note: master has only peer=postfix-master and already dropped the path-based rules.