Add 'smstools' role

This role configures TCP -> SMS and mail -> SMS gateway.
Fix problem with IPv6 address in /etc/hosts.allow

IPv6 addresses in /etc/hosts.(allow|deny) need to be enclosed in square
brackets to be correctly parsed by xinetd.
Fix dependency variable support in Postfix role

Postfix dependency template didn't add empty values in
'postfix_dependent_maincf' list to generated configuration table, which
are supported by Postfix role, and allow you to specify which lists from
'postfix_dependent_lists' hash variable should be checked for non-empty
lists and included in Postfix main.cf configuration file. This patch
fixes that problem.
Check if modem device exists in smstools script
Merge pull request #104 from drybjed/smstools

Add 'smstools' role and fix related bugs in other roles