Remove libthread_db -> remove thread_stratum? [was Re: Cannot execute this command without a live selected thread.]

[Doug Evans <dje@google.com>]

[note subject change]

Doug Evans writes:
 > Pedro Alves writes:
 >  > > Not all targets use ptid.lwp.
 >  > 
 >  > All process_stratum targets do.
 > 
 > windows-nat.c doesn't
 > (at least I remember seeing all calls to ptid_build there
 > passing 0 for lwp).
 > Could be missing something of course.
 > 
 >  >     I believe that on the GDB side too, it's best that we standardize on
 >  >     process_stratum targets using the ptid.lwp field to store thread ids
 >  >     anyway.  The idea being leave the ptid.tid field free for any
 >  >     thread_stratum target that might want to sit on top.
 > 
 > The language in the comment in ptid.h waffles a bit:
 > 
 >    process_stratum targets that handle threading themselves should
 >    prefer using the ptid.lwp field, leaving the ptid.tid field for any
 >    thread_stratum target that might want to sit on top.
 > 
 > Can we make this more of a rule than just a "should prefer"?
 > [and fix targets to follow]

Oh, btw, another question I've been wanting to ask ...

One goal we have is to remove libthread_db on linux.
There are two reasons we still have it, pthread_t and thread local vars,
though those can be solved.

Long term, at least in linux-land,
do we still want to keep thread_stratum?
Knowing the answer to this will help save some typing.
[If one did want to remove thread_stratum for linux
there's still a need to support older systems.
But on future newer systems without libthread_db,
what would thread_stratum look like?]