Added some defensive code to net/socket functions

[Christopher Faylor]

As I was in the process of adding the fd protection code that was just
mentioned in the cygwin mailing list, I decided to add the normal buffer
checking defensive code to most of the net/socket functions.  I've
verified that sshd, inetd, and telnetd still work, but I'd appreciate it
if I could get some assurance that I haven't broken anything else.

The checking in net.cc was a lot trickier than I had anticipated so
I could easily have gotten something wrong.

If someone could verify my changes, it would be great.

And, if anyone wants to complete the last few functions that I didn't
get to, that would be great, too.

'cvs diff -up -r1.86 -r1.87' should show you my changes.

cgf