add support for "unspecified" configuration entries

[anonymous]

add support for "unspecified" configuration entries.

Imagine I configure some setting properly only for debian and macosx computers. I would like bcfg2 to complain if (someday) I bind (by mistake) some computer that is neither debian nor macosx. Adding an "unspecifed".

for instance:

<FileInfo>
    <Group name='debian'>
      <Permissions owner='root' group='root' perms='0644'/>
    </Group>
    <Group name='macosx'>
      <Permissions owner='root' group='wheel' perms='0644'/>
    </Group>
    <unspecified/>
</FileInfo>

This would allow the one that writes some configuration specification to prevent another one to use in an unwanted way.