Handle EAGAIN and EWOULDBLOCK more gracefully when dealing with safe_read().
All occurrences of safe_read() should only be invoked on fds that have signaled
ready-to-read state via the epoll() mechanism, so this change should not
result in any observable difference, but it is best to be safe.
Additionally, a constant stack variable is converted to an equivalent
macro define for cleanliness.
Finally, print the error type encountered if reading data from an ARP response
fails with a read error.
