on descriptors, this can be achieved thanks to the new getdtablecount()
system call. application may provide a reserve count to ensure that the
recvmsg() call is not called when they don't have enough descriptors to
work properly.
change the API so that transient errors that can be retried immediately
are retried within the function right away, whereas transient errors for
which the application may want to take action will set errno to EAGAIN.
ok deraadt@ and henning@