Remove usage of find_inferior in iterate_over_lwps

Remove usage of find_inferior in iterate_over_lwps

Replace find_inferior with find_thread.  Since it may be useful in the
future, I added another overload to find_thread which filters based on a
ptid (using ptid_t::matches), so now iterate_over_lwps doesn't have to
do the filtering itself.  iterate_over_lwps_filter is removed and
inlined into iterate_over_lwps.

gdb/gdbserver/ChangeLog:

	* gdbthread.h (find_thread): Add overload with ptid_t filter.
	* linux-low.c (struct iterate_over_lwps_args): Remove.
	(iterate_over_lwps_filter): Remove.
	(iterate_over_lwps): Use find_thread.