Add overloads of for_each_thread/find_thread that filter on pid
It happens often that we want to iterate or find threads restricted to a
given pid. I think it's worth having an overload to help with this.
Right now there is a single user of each of the find_thread and
for_each_thread overload, but as we replace the usages of find_inferior
with for_each_thread/find_thread, more usages will pop up.
gdb/gdbserver/ChangeLog:
* gdbthread.h (find_thread, for_each_thread): New functions.
* inferiors.c (thread_of_pid): Remove.
(find_any_thread_of_pid): Use find_thread.
* linux-low.c (num_lwps): Use for_each_thread.