ptrace: Add PT_POKE

PT_POKE writes a single word to the tracee's address space.

Some caveats:
- If the user requests to write to an address in a read-only region, we
temporarily change the page's protections to allow it.

- If the user requests to write to a region that's backed by a
SharedInodeVMObject, we replace the vmobject with a PrivateIndoeVMObject.

Kernel/UnixTypes.h

@@ -555,3 +555,4 @@ struct rtentry {
 #define PT_GETREGS 5
 #define PT_DETACH 6
 #define PT_PEEK 7
+#define PT_POKE 8