SystemMonitor: Show process unveil() state as "Veil"

A process has one of three veil states:

- None: unveil() has never been called.
- Dropped: unveil() has been called, and can be called again.
- Locked: unveil() has been called, and cannot be called again.

Libraries/LibCore/CProcessStatisticsReader.h

@@ -64,6 +64,7 @@ struct CProcessStatistics {
     String name;
     String tty;
     String pledge;
+    String veil;
     size_t amount_virtual;
     size_t amount_resident;
     size_t amount_shared;