Open SBBS_SSJS.*.html response file(s) only when actually writing data

The first write to a the temporary SBBS_SSJS.*.html file will open the file.
This should reduce the number of 0-byte files left laying around in the the
temp directory, which shouldn't be happening in the first place.

Also:
Fixed bug noticed in temp file clean up loop:  POST data files would *also*
be retained when the DEBUG_SSJS option flag is set.

Also:
Replace some unsafe string operations with safe equivalents.

Happy Thanksgiving Nelgin!