MFC r234098 (jhb) and r255047.

r234098:
Add media types for 40G media that might be used with FreeBSD.

r255047:
Add a routine for attaching an mbuf to a buffer with an external
refcount. This one is willing to work with buffers that may already be
referenced. MEXTADD/m_extadd are suitable only for the first attachment
to a cluster -- they initialize the refcount to 1.