Fix return type of extattr_set_* and fix rmextattr(8) utility.

extattr_set_{fd,file,link} is logically a write(2)-like operation and
should return ssize_t, just like extattr_get_*.  Also, the user-space
utility was using an int for the return value of extattr_get_* and
extattr_list_*, both of which return an ssize_t.

MFC after:	1 week
Regen.

MFC after:	1 week
Add VIMAGE to NOTES.

Reviewed by:	zec
Add a missing unmap; if we're freeing this mbuf then we must
really both sync/unmap the dmamap before freeing it.
Some TX dmamap cleanups.

* Don't use BUS_DMA_ALLOCNOW for descriptor DMA maps; we never use
  bounce buffers for the descriptors themselves.

* Add some XXX's to mark where the ath_buf has its mbuf ripped from
  underneath it without actually cleaning up the dmamap.  I haven't
  audited those particular code paths to see if the DMA map is guaranteed
  to be setup there; I'll do that later.

* Print out a warning if the descdma tidyup code is given some descriptors
  w/ maps to free.  Ideally the owner will free the mbufs and unmap
  the descriptors before freeing the descriptor/ath_buf pairs, but
  right now that's not guaranteed to be done.

Reviewed by:	scottl (BUS_DMA_ALLOCNOW tag)
Mark a couple of places where I think the dmamap isn't being unmapped
before the TX path is being aborted.

Right now it's in the TDMA code and I can live with that; but it really
should get fixed.

I'll do a more thorough audit of this code soon.