Avoid shadowing in gdbserver

Avoid shadowing in gdbserver

This fixes a few instances of shadowing in gdbserver.  These are all
simple fixes.

gdb/gdbserver/ChangeLog
2018-10-04  Tom Tromey  <tom@tromey.com>

	* server.c (handle_status): Rename inner "thread".
	(process_serial_event): Declare "res" in 'm' case.
	* linux-low.c (last_thread_of_process_p, find_lwp_pid)
	(iterate_over_lwps): Rename inner "thread".
	(linux_qxfer_libraries_svr4): Rename inner "len".
	* gdbthread.h (find_thread_in_random): Rename inner "thread".