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.

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