Handle partial writes on non-blocking sockets

Handle partial writes on non-blocking sockets

Now that we're using krb5_net_write() with non-blocking sockets in
ipropd_master, we MUST correctly account for partial writes.

Therefore, roken net_write() called from krb5_net_write() now
returns the number of bytes written when the socket error was
EWOULDBLOCK (or EAGAIN).

Also, fix potential issue on Windows, where errno was used instead
of rk_SOCKET_ERRNO whether or not we used _write() or send().