patch for icmp.h

[John Paul Morrison]

1. OK, so the preferred solution to use the BSD header files?

2. #define _CYGWIN_ICMP_H looks like the right convention

3. AFAIK, you need admin rights in Unix to use raw/icmp sockets, at 
least I thought that was the reason ping and traceroute were setuid.
I think the details are good to document and are certainly helpful for 
porting, but I'm assuming they wouldn't change the header files.