postfix: broken mynetworks parameter default value

Fixed an issue with the default parameter value.

1. By default, the init script set ip address instead of network address in the parameter value (i.e. 192.168.1.1/24 instead of 192.168.1.0/24)
2. Dynamic address were not caught by the init script

As a result, if the mynetworks parameter was left default, Postfix would have rejected the mails with the following message in the logs:

Sat Dec  6 23:26:45 2014 mail.info postfix/smtpd[22806]: NOQUEUE: reject: RCPT from unknown[104.167.106.30]: 451 4.3.0 <ds_gitcommit@shulyaka.org.ru>: Temporary lookup failure; from=<delivery@mx.sailthru.com> to=<ds_gitcommit@shulyaka.org.ru> proto=ESMTP helo=<mx-indiegogo-b.sailthru.com>
Sat Dec  6 23:26:45 2014 mail.warn postfix/smtpd[22806]: warning: non-null host address bits in "127.0.0.1/8", perhaps you should use "127.0.0.0/8" instead

This commit fixes that behavior for the new installations.

Signed-off-by: Denis Shulyaka <Shulyaka@gmail.com>