Fix ignore-hosts-file operation

This fixes the following error:
Global symbol "$ignore_hosts_file" requires explicit package name at /opt/vyatta/sbin/vyatta-dns-forwarding.pl line 77.
Global symbol "$ignore_hosts_file" requires explicit package name at /opt/vyatta/sbin/vyatta-dns-forwarding.pl line 86.
Global symbol "$ignore_hosts_file" requires explicit package name at /opt/vyatta/sbin/vyatta-dns-forwarding.pl line 99.
Execution of /opt/vyatta/sbin/vyatta-dns-forwarding.pl aborted due to compilation errors.
Fix dnsmasq.conf generation for ignore-hosts-file

The ignore-hosts-file option did not work; because it's a boolean it didn't return any value and was always undefined when the test was done.  Using $config->exists() fixes this.
Merge pull request #15 from paulgear/helium

Bug #255: Correct ignore-hosts-file operation