add modem emulation (for online gaming)

some games needs a server to be really playable, for exemple:

• Digital Keiba Shinbun: My Trackman
• Grauen no Torikago Kapitel 1 - 6
• Planet Ring
• Sakura Taisen Online - Teito no Yuugana Hibi
• Sakura Taisen Online - Paris no Nagai Hibi

but many other games can also be played online with private servers http://www.dreamcastlive.net/blogs/tag/private%20server