bug#21094: cp: add option to sort when copying

[Steve Russell]

When performing a recursive copy of a large number of files it would be helpful 
to have an option to copy the files in each directory in sorted order of the 
file names. There are a couple of use cases for this.
 
The first is to be able to easily observe the progress of the copy using the -v 
option. Currently, the "random" order of the copy means I can not tell how far 
the copy has progressed. On occasions I have seen cp -v display a top level 
directory starting with 'z' and assumed the copy was almost done only to see it 
start copying files from another directory earlier in the alphabet.
 
The second use case is sometimes I would like to start working with the 
destination files before the copy has completed. For example if I see the 
destination has the top level directory "foo", an ordered copy means I can work 
with directories that are alphabetically before "foo" knowing that they will 
not change.

Another minor use case is some mp3 players ignore the file names and metadata 
and always play tracks in the order they appear in the directory. Usually the 
filesystems used by these players will order directories in the same order 
files are added. For content like eBooks where track order is critical, a 
sorted copy would be a useful workaround for the limitations of these players.