Pass thread_info pointer to various inferior control functions

Pass thread_info pointer to various inferior control functions

[ Migrating this from Gerrit: https://gnutoolchain-gerrit.osci.io/r/c/binutils-gdb/+/321 ]

I noticed that some functions in infcmd and infrun call each other and
all call inferior_thread, while they could just get the thread_info
pointer from their caller.  That means less calls to inferior_thread, so
less reliance on global state, since inferior_thread reads
inferior_ptid.

The paths I am unsure about are:

  - fetch_inferior_event calls...
  - step_command_fsm::should_stop calls...
  - prepare_one_step

and

 - process_event_stop_test calls...
 - set_step_info

Before this patch, prepare_one_step gets the thread pointer using
inferior_thread.  After this patch, it gets it from the
execution_control_state structure in fetch_inferior_event.  Are we sure
that the thread from the execution_control_state structure is the same
as the one inferior_thread would return?  This code path is used when a
thread completes a step, but the user had specified a step count (e.g.
"step 5") so we decide to do one more step.  It would be strange (and
even a bug I suppose) if the thread in the ecs structure in
fetch_inferior_event was not the same thread that is prepared to stepped
by prepare_one_step.  So I believe passing the ecs thread is fine.

The same logic applies to process_event_stop_test calling
set_step_info.

gdb/ChangeLog:

	* infrun.h: Forward-declare thread_info.
	(set_step_info): Add thread_info parameter, add doc.
	* infrun.c (set_step_info): Add thread_info parameter, move doc
	to header.
	* infrun.c (process_event_stop_test): Pass thread to
	set_step_info call.
	* infcmd.c (set_step_frame): Add thread_info pointer, pass it to
	set_step_info.
	(prepare_one_step): Add thread_info parameter, pass it to
	set_step_frame and prepare_one_step (recursive) call.
	(step_1): Pass thread to prepare_one_step call.
	(step_command_fsm::should_stop): Pass thread to
	prepare_one_step.
	(until_next_fsm): Pass thread to set_step_frame call.
	(finish_command): Pass thread to set_step_info call.