Refactor WLAN PSK retrieval code

- Reduce platform specific code in ImageWriter class,
  and move that to seperate classes.
- Use API calls to get current SSID on Windows and Linux instead
  of launching command line utilities.

src/mac/macwlancredentials.cpp

@@ -0,0 +1,73 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ * Copyright (C) 2023 Raspberry Pi Ltd
+ */
+
+#include "macwlancredentials.h"
+#include <security/security.h>
+#include <QProcess>
+#include <QRegularExpression>
+
+QByteArray MacWlanCredentials::getSSID()
+{
+    /* FIXME: find out the proper API call to get SSID instead of calling command line utilities */
+    QString program, regexpstr;
+    QStringList args;
+    QProcess proc;
+    program = "/System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport";
+    args << "-I";
+    regexpstr = "[ \t]+SSID: (.+)";
+
+    proc.start(program, args);
+    if (proc.waitForStarted(2000) && proc.waitForFinished(2000))
+    {
+        QRegularExpression rx(regexpstr);
+        const QList<QByteArray> outputlines = proc.readAll().replace('\r', "").split('\n');
+
+        for (const QByteArray &line : outputlines) {
+            QRegularExpressionMatch match = rx.match(line);
+            if (match.hasMatch())
+            {
+                _ssid = match.captured(1).toLatin1();
+                break;
+            }
+        }
+    }
+
+    return _ssid;
+}
+
+QByteArray MacWlanCredentials::getPSK()
+{
+    SecKeychainRef keychainRef;
+    QByteArray psk;
+
+    if (_ssid.isEmpty())
+        getSSID();
+    if (_ssid.isEmpty())
+        return psk;
+
+    if (SecKeychainOpen("/Library/Keychains/System.keychain", &keychainRef) == errSecSuccess)
+    {
+        UInt32 resultLen;
+        void *result;
+        if (SecKeychainFindGenericPassword(keychainRef, 0, NULL, _ssid.length(), _ssid.constData(), &resultLen, &result, NULL) == errSecSuccess)
+        {
+            psk = QByteArray((char *) result, resultLen);
+            SecKeychainItemFreeContent(NULL, result);
+        }
+        CFRelease(keychainRef);
+    }
+
+    return psk;
+}
+
+WlanCredentials *WlanCredentials::_instance = NULL;
+WlanCredentials *WlanCredentials::instance()
+{
+    if (!_instance)
+        _instance = new MacWlanCredentials();
+
+    return _instance;
+}
+