introduce os.EOF and io.ErrUnexpectedEOF.
remove io.ErrEOF.
rename io.FullRead to io.ReadFull, to match
   ReadAtLeast and ReadAll.
remove io.FullReader, because it is now unused.

R=r
DELTA=295  (88 added, 105 deleted, 102 changed)
OCL=30544
CL=30588