append/clobber flag for save, stringout, with_stdout

Hello,

I have implemented a flag file_output_append to tell save, stringout, and
with_stdout whether to append or clobber their output file.
The default is false (clobber).

This was inspired by a recent question about stringout and a
comment from Albert Reiner.

I believe that these are the only functions in maxima/src/ which
can usefully distinguish append from clobber. There are some
other functions which should always clobber (plotting and
translation) or always append (tex and appendfile).

If this implementation seems OK, I'll update the texinfo docs
and also make numericalio pay attention to the flag.

The implementation was tested on GCL, Clisp, and SBCL on
Linux and worked as expected.

Hope this helps,
Robert Dodier