Return ENETDOWN instead of ENOENT when all lagg(4) links are
inactive when upper layer tries to transmit packet.  This
gives better feedback and meaningful errors for applications.

MFC after:	2 weeks
Reviewed by:	thompsa
Clean up -Wheader-guard warnings.

Submitted by:	<dt71@gmx.com>
MFC after:	3 days
X-MFC with:	r251848
Fix header guards.

This was ready about the same time as r251862 so just make one final cleanup

Submitted by:	dt71@gmx.com