In rare circumstances, such as checksum errors, some network stacks

In rare circumstances, such as checksum errors, some network stacks
can flag an fd for reading in select() and still block when the fd is
read.  Set all sockets non-blocking to prevent hangs when this occurs.
(We don't actually handle the resulting EWOULDBLOCK or EAGAIN errors,
so the rare cases will appear as communication failures and we will
close the socket.  This is already the case for TCP sockets and
probably isn't a big deal.)

ticket: 6933

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@25048 dc483132-0cff-0310-8789-dd5450dbe970