fixes for CMU User's Manual

Hi, CMUCL / rtoy

Recently I tried UDP networking in CMUCL then found some mistakes in  
CMU User's Manual:

1. Three functions INET-SENDTO, INET-RECVFROM, INET-SHUTDOWN is not  
external symbol of UNIX package but EXTENSIONS.

2. INET-SENDTO should have a ADDR argument.

3. INET-RECVFROM maybe need more document, or I don't know how to get  
receive data from a socket.

Attach is my change to internet.tex, please review it.

Regards,

Chun Tian (binghe)