Replace depreciated com2str in zpout.m

[John W. Eaton]

On 31-Jan-2009, Ben Abbott wrote:

| I don't see zpout in the developer's sources. In fact, I don't the the  
| control directory at all.
| 
| ... also Octave-Forge still links to it
| 
| 	http://octave.sourceforge.net/doc/f/zpout.html
| 
| The link on that page is below.
| 
| 	http://velveeta.che.wisc.edu/cgi-bin/hgwebdir.cgi/octave/file/tip//scripts/control/system/zpout.m?rev=HEAD&content-type=text/plain

The control, finance, and quaternion packages are more or less in
limbo until 3.2 is released, though we will probably make them
available in some way for the snapshots leading up to the 3.2 release.
I think David Bateman has the latest sources.  I think we need to put
them somewhere public so people who are using the development sources
or one of the upcoming snapshots can test them with the current Octave.

jwe