dnsmasq: bump to 2.51 and add IDN option

Closes #717

Dnsmasq 2.51 introduces IDN support, add it to the package.
Newsworthy items:

* Add support for internationalised DNS (IDN).
* Improved upstream nameserver checks for slow nameservers.
* Improved TFTP support (win/dos filenames, new tftpserver-address option
  for DHCP).

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>