[Mimedefang] documentation bug/omission for newbies

[starlight.2016q2 at binnacle.cx]

Here's an improvment to the basic body-scan snippet so it does not attempt to scan a non-text first/body mime part:

my $bdyscn_flag = 0;
my $part0 = $entity->parts(0);
if (defined $part0) {
    my ($type, $subtype) = split('/', $part0->effective_type);
    my $bh = $type eq 'text' ? $part0->bodyhandle : undef;
    if (defined $bh && (my $bio = $bh->open("r"))) {
        while (defined($_ = $bio->getline)) {
            chomp;
            if (m/MATCH SOME TEXT/) {
                $bdyscn_flag = 1; last
            }
        }
        $bio->close;
    }
}
if ($bdyscn_flag) {
   # do something
}