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.

Applications/SystemMonitor/ProcessModel.h

@@ -61,6 +61,7 @@ public:
         CleanInode,
         PurgeableVolatile,
         PurgeableNonvolatile,
+        Veil,
         Pledge,
         Syscalls,
         InodeFaults,
@@ -100,6 +101,7 @@ private:
         String state;
         String user;
         String pledge;
+        String veil;
         u32 priority;
         u32 effective_priority;
         size_t amount_virtual;