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.

Libraries/LibC/sys/ptrace.h

@@ -37,6 +37,7 @@ __BEGIN_DECLS
 #define PT_GETREGS 5
 #define PT_DETACH 6
 #define PT_PEEK 7
+#define PT_POKE 8
 
 int ptrace(int request, pid_t pid, void* addr, int data);