Make step_start_function be per thread

Make step_start_function be per thread

I noticed that step_start_function is still a global, while it
obviously should be a per-thread field.

gdb/ChangeLog:
2015-03-24  Pedro Alves  <palves@redhat.com>

	* infrun.c (step_start_function): Delete and ...
	* gdbthread.h (struct thread_control_state) <step_start_function>:
	... now a field here.
	* infrun.c (clear_proceed_status_thread): Clear the thread's
	step_start_function.
	(proceed, process_event_stop_test, print_stop_event): Adjust.