sample configurations for rsyslog, postfix are outdated

[Markus Ueberall]

1) the below was tested on Ubuntu Xenial Xerus (development branch, kept up-to-date)

2) monit package version: 1:5.16-2

3) expected behaviour: enabling the respective included configuration sample 'just works' when the package in question is installed as well

4) actual behaviour: monit would refuse to start or properly monitor postfix, rsyslog(d):
    - /etc/monit/conf-available/postfix refers to /usr/lib/postfix/master which moved to /usr/lib/postfix/sbin/master (Xenial Xerus, development branch)
    - /etc/monit/conf-available/rsyslog refers to /var/log/messages which is either empty nowadays and/or has wrong 0644 permission (after upgrade) or has not been created in the first place (Xenial Xerus, development branch)