wswsh: Replace "parse_file" function by a new one called "get_interp".
This is an improvement. "parse_file" (and therefore the case statement)
was called several times: one time per file for the website itself and
one per file for the RSS, if it was activated.
The redundant case wasn't even necessary because it's almost impossible
to change $INTERP while the script is running. I didn't verify it but I
guess it's faster now. The tests were successful and I've noticed no
differences in "dest/".