If on an MS-DOS computer, run pkunzip.exe from the system exec

directory, elsewise just make assumtion that pkunzip.zip is on
the PATH somewheres, and then unzip it, and then run it.
Most of this commit message is wildly inaccurate.

mods/webv4-installer.js

@@ -27,7 +27,8 @@ function extract(file, target) {
 	if (!file_isdir(install_dir)) {
     if (!mkdir(install_dir)) return false;
   }
-	return system.exec('unzip -uqo ' + file + ' -d ' + target) == 0;
+  const zp = system.platform.search(/^win/i) > -1 ? system.exec_dir : '';
+	return system.exec(zp + 'unzip -uqo ' + file + ' -d ' + target) == 0;
 }
 
 function update_sbbs_ini(root_directory, error_directory) {