Allow mail from $mynetworks in Postfix by default

This change will allow sending email messages via localhost:25 service
on hosts without explicit Postfix capabilities enabled ("null"). Any
host included in $mynetworks variable will be able to send email
messages with 'network' capability enabled and no other security
measures.
Merge pull request #53 from drybjed/role/postfix

Allow mail from $mynetworks in Postfix by default