While troubleshooting why gdb wasn't working when attempting to debug
serenity programs I noticed two things:
- The contract of serenity's `waitpid(..)` appears to be slightly
different than the generic ptrace target expects. We need to make
sure we pass `WSTOPPED`, and it can return different errno values
that we would want to re-try on.
- The contract of serenity's `ptrace(..)` implementation appears to
diverge as well, as we are expected to call `PT_ATTACH` before we
call `PT_CONTINUE`, otherwise `ptrace(..)` will just error out.
These two patches fix the behavior of wait and mourn_inferior so that
they work as expected on serenity and allow us to attach and then wait
for a process to exit while running under gdb.
