title: find out AF_UNIX + SOCK_SEQPACKET maximum message size
body: <p>I wonder if there is any possibility to find out the max length of <code>SEQPACKET</code> except experimental (a-la <code>for( i=0; i&lt;100...00; i++ ) send( ... );</code>).</p>
  <p>And, the second question:</p>
  <p>If I received <code>errno == EMSGSIZE</code> trying to send <code>AF_UNIX</code> <code>SEQPACKET</code>, is it guaranteed that it's because of max message size, or could there be some other reasons?</p>