fs: clarify fs.link and fs.linkSync arguments

Updates the argument names `srcpath` and `dstpath` to match the more
descriptive `existingPath` and `newPath` in the documentation.

PR-URL: #9145
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>

lib/fs.js

@@ -1056,24 +1056,24 @@ fs.symlinkSync = function(target, path, type) {
                          type);
 };
 
-fs.link = function(srcpath, dstpath, callback) {
+fs.link = function(existingPath, newPath, callback) {
   callback = makeCallback(callback);
-  if (!nullCheck(srcpath, callback)) return;
-  if (!nullCheck(dstpath, callback)) return;
+  if (!nullCheck(existingPath, callback)) return;
+  if (!nullCheck(newPath, callback)) return;
 
   var req = new FSReqWrap();
   req.oncomplete = callback;
 
-  binding.link(pathModule._makeLong(srcpath),
-               pathModule._makeLong(dstpath),
+  binding.link(pathModule._makeLong(existingPath),
+               pathModule._makeLong(newPath),
                req);
 };
 
-fs.linkSync = function(srcpath, dstpath) {
-  nullCheck(srcpath);
-  nullCheck(dstpath);
-  return binding.link(pathModule._makeLong(srcpath),
-                      pathModule._makeLong(dstpath));
+fs.linkSync = function(existingPath, newPath) {
+  nullCheck(existingPath);
+  nullCheck(newPath);
+  return binding.link(pathModule._makeLong(existingPath),
+                      pathModule._makeLong(newPath));
 };
 
 fs.unlink = function(path, callback) {