RFC strace via gdbserver

[Stan Cox]

I just made one small change.  I didn't want to start using the 
vContc;c:ppid.tid form until I saw the completed syscall where the tid 
changed.  For recent linux this is typically set_robust_list.  So once 
the syscall return is seen it starts using the pid.tid form.  This was 
already there in a simple form but this makes it per tid.  I added a 
field to struct tcb called gdb_use_pid_tid to do this.

I rebranched from the https://github.com/cuviper/strace.git default 
branch to:
  remote.origin.url=git at github.com:stanfordcox/strace.git
  * gdbserver0         9b817e8 Use a per struct tcb value to determine
which is now the default branch