This was a missing feature in the PT_TRACEME command.
This feature allows the tracer to interact with the tracee before the
tracee has started executing its program.
It will be useful for automatically inserting a breakpoint at a
debugged program's entry point.
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.
