GNU/Linux calls tgid, "thread group ID". When we support
attaching to threads, the original thread may not be the correct
thread. We would have to get the process ID from /proc for NPTL.
- For LinuxThreads we could do something similar: follow the chain
- of parent processes until we find the highest one we're attached
- to, and use its tgid.
This isn't the only place in gdbserver that assumes that the first
process in the list is the thread group leader. */