Linux/FreeBSD: Prevent mounting volumes on system directories and PATH (CVE-2025-23021, reported by SivertPL @__tfr)

Added security checks to prevent mounting VeraCrypt volumes on system directories (like /usr/bin) or directories in the user's PATH, which could theoretically allow execution of malicious binaries instead of legitimate system binaries.

Key changes:
- Block mounting on protected system directories (/usr, /bin, /lib, etc.)
  This restriction cannot be overridden
- Block mounting on directories present in user's PATH environment variable
  This can be overridden with --allow-insecure-mount flag
- Add visual warnings (red border, "[INSECURE MODE]") when mounting on PATH directories is allowed
- Handle symlinks properly when checking paths
- Add new error messages for blocked mount points

To override PATH-based restrictions only (system directories remain protected):
veracrypt --allow-insecure-mount [options] volume mountpoint

Security Impact: Low to Medium
The attack requires either:
- User explicitly choosing a system directory as mount point instead of using VeraCrypt's default mount points
- Or attacker having both filesystem access to modify favorites configuration AND knowledge of the volume password
Default mount points are not affected by this vulnerability.

Security: CVE-2025-23021

src/Main/GraphicUserInterface.cpp

@@ -1082,7 +1082,12 @@ namespace VeraCrypt
 #endif
 
 			mMainFrame = new MainFrame (nullptr);
-
+#if defined(TC_UNIX)
+			if (CmdLine->ArgAllowInsecureMount)
+			{
+				mMainFrame->SetTitle (mMainFrame->GetTitle() + wxT(" ") + LangString["INSECURE_MODE"]);
+			}
+#endif
 			if (CmdLine->StartBackgroundTask)
 			{
 				UserPreferences prefs = GetPreferences ();