postfix 3.1.0: postinst contains broken tests

[Jim Klimov]

Ubuntu 16.04 includes postfix-3.1.0-3 which fails for us installing in an OBS buildroot (so a very minimal system), with the issue apparently fixed in 3.1.3 : https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=842948

FWIW, the exact error message we have is:

````
[ 148s] [272/285] installing postfix-3.1.0-3
[ 149s] Adding group `postfix' (GID 103) ...
[ 149s] Done.
[ 150s] Adding system user `postfix' (UID 100) ...
[ 150s] Adding new user `postfix' (UID 100) with group `postfix' ...
[ 150s] Not creating home directory `/var/spool/postfix'.
[ 150s] Creating /etc/postfix/dynamicmaps.cf
[ 150s] Adding group `postdrop' (GID 104) ...
[ 150s] Done.
[ 152s] setting myhostname: .UNKNOWN
[ 152s] setting alias maps
[ 152s] setting alias database
[ 152s] mailname is not a fully qualified domain name. Not changing /etc/mailname.
[ 152s] setting destinations: $myhostname, localdomain, localhost, localhost.localdomain, localhost
[ 152s] setting relayhost:
[ 152s] setting mynetworks: 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
[ 152s] setting mailbox_size_limit: 0
[ 152s] setting recipient_delimiter: +
[ 152s] setting inet_interfaces: all
[ 154s] setting inet_protocols: all
[ 154s] /etc/aliases does not exist, creating it.
[ 154s] WARNING: /etc/aliases exists, but does not have a root alias.
[ 154s] Postfix is now set up with a default configuration. If you need to make
[ 154s] changes, edit
[ 154s] /etc/postfix/main.cf (and others) as needed. To view Postfix configuration
[ 154s] values, see postconf(1).
[ 154s] After modifying main.cf, be sure to run '/etc/init.d/postfix reload'.
[ 154s] Running newaliases
[ 156s] newaliases: warning: valid_hostname: misplaced delimiter: .UNKNOWN
[ 156s] newaliases: fatal: file /etc/postfix/main.cf: parameter myhostname: bad parameter value: .UNKNOWN
[ 156s] dpkg: error processing package postfix (--install):
[ 156s] subprocess installed post-installation script returned error exit status 75
[ 156s] Errors were encountered while processing:
[ 156s] postfix
[ 156s] exit ...
[ 156s] Halting runtime env under BUILD_ROOT='/var/cache/obs/worker/root_6' ...
````

Due to this, we can not build packages whose dependency chain includes postfix.

[Scott Kitterman (kitterman)]

That's got it properly triaged. I only work on postfix in Debian, so I can't say if this will be fixed or not in Ubuntu.