Avoid setting $PWD as "."

That would cause strange result. For example, run "konsole --workdir .",
then bash or zsh would report its current dir as "."

This is a follow up of commit 790e835c6f

src/Pty.cpp

@@ -134,7 +134,10 @@ char Pty::eraseChar() const
 void Pty::setInitialWorkingDirectory(const QString& dir)
 {
     setWorkingDirectory(dir);
-    setEnv("PWD", dir);
+
+    // setting PWD to "." will cause problem for bash & zsh
+    if ( dir != "." )
+        setEnv("PWD", dir);
 }
 
 void Pty::addEnvironmentVariables(const QStringList& environment)