QLockFile: don't use QCoreApplication::applicationName()

That was stupid for two reasons. First, applicationName() can be changed
by the user by calling QCoreApplication::setApplicationName(), so if
that happens, we won't be able to properly identify a lock belonging to a
given application because the name changed.

Second, because applicationName() is not what we compare to. Instead,
let's use processNameByPid() on both content creation and verification.

[ChangeLog][QtCore][QLockFile] Fixed a bug that would cause QLockFile
mis-identify valid lock files as stale if the application name was set
with QCoreApplication::setApplicationName().

Change-Id: I0b48fc8e90304e0dacc3fffd14e912a5c98c87e7
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

src/corelib/io/qlockfile.cpp

@@ -330,7 +330,7 @@ QByteArray QLockFilePrivate::lockFileContents() const
 {
     // Use operator% from the fast builder to avoid multiple memory allocations.
     return QByteArray::number(QCoreApplication::applicationPid()) % '\n'
-            % QCoreApplication::applicationName().toUtf8() % '\n'
+            % processNameByPid(QCoreApplication::applicationPid()).toUtf8() % '\n'
             % machineName().toUtf8() % '\n';
 }