Fix rs_file_copy_cb, which needs to know the position at which to copy.  Show fd# if copy fails.
Fix rs_file_copy_cb, which needs to know the position at which to
copy.  Show fd# if copy fails.

Doc.

pauloppenheim-liveloop/librsync,pauloppenheim-liveloop/librsync-winbuild,librsync/librsync,sourcefrog/librsync,sourcefrog/librsync,librsync/librsync,VictorDenisov/librsync,yrgoldteeth/librsync,binondord/librsync,yrgoldteeth/librsync,potatogim/librsync,binondord/librsync,agrandville/librsync,pauloppenheim-liveloop/librsync-winbuild,dbaarda/librsync,Salamek/librsync,pauloppenheim-liveloop/librsync,yrgoldteeth/librsync,yrgoldteeth/librsync,librsync/librsync,dbaarda/librsync,binondord/librsync,VictorDenisov/librsync,dbaarda/librsync,sourcefrog/librsync,agrandville/librsync,dbaarda/librsync,Salamek/librsync,VictorDenisov/librsync,VictorDenisov/librsync,pauloppenheim-liveloop/librsync,agrandville/librsync,librsync/librsync,sourcefrog/librsync,pauloppenheim-liveloop/librsync-winbuild,Salamek/librsync,potatogim/librsync,potatogim/librsync