secureserver.pl: fix stunnel path quoting

secureserver.pl: fix stunnel path quoting

- Store the stunnel path in the private variable $stunnel unquoted and
  instead quote it in the command strings.

Prior to this change the quoted stunnel path was passed to perl's file
operators which cannot handle quoted paths. For example:

$stunnel = "\"/C/Program Files (x86)/stunnel/bin/tstunnel\"";
if(-x $stunnel or -x "$stunnel")
# false even if path exists and is executable

Our other test scripts written in perl, unlike this one, use servers.pm
which has a global $stunnel variable with the path stored unquoted and
therefore those scripts don't have this problem.

Closes https://github.com/curl/curl/pull/11721