ini_file: Allow a "global" section without header.

Some INI-style files can have a "global properties", variable settings
that come first in the file before any section header.  Here we add
support in the ini_file definition for generating such settings, by
putting them in a section named "" (the empty string).  So for example

    cfgfile::ini_file {
        '/etc/smurf.ini':
            settings => {
                'smurfs' => { 'colour' => 'blue', },
                '' => { 'creator' => 'Pierre', },
            };
    }

will generate a file with the content

    creator = Pierre

    [smurfs]
    colour = blue