Add networking tweaks to the setup

[chros73]

Add networking tweaks to the setup, including:

• system wide settings
• rtorrent config entries in the form of: network.receive_buffer.size and network.send_buffer.size

The latter 2 adjust the send and receive buffer size for socket. Disabled by default (0), this means the default is used by OS (you have to modify the system wide settings!!!) (send_buffer_size, receive_buffer_size)
Increasing buffer sizes may help reduce disk seeking, connection polling as more data is buffered each time the socket is written to. It will result higher memory usage (not by rtorrent process!).

These values has to be adjusted in the system sysctl.conf file as well.