Make common code handle target_terminal_* idempotency

Make common code handle target_terminal_* idempotency

I found a place that should be giving back the terminal to the target,
but only if the target was already owning it.  So I need to add a
getter for who owns the terminal.

The trouble is that several places/target have their own globals to
track this state:

 - inflow.c:terminal_is_ours
 - remote.c:remote_async_terminal_ours_p
 - linux-nat.c:async_terminal_is_ours
 - go32-nat.c:terminal_is_ours

While one might think of adding a new target_ops method to query this,
conceptually, this state isn't really part of a particular target_ops.
Considering multi-target, the core shouldn't have to ask all targets
to know whether it's GDB that owns the terminal.  There's only one GDB
(or rather, only one top level interpreter).

So what this comment does is add a new global that is tracked by the
core instead.  A subsequent pass may later remove the other globals.

Tested on x86_64 Fedora 20, native and gdbserver.

gdb/
2014-10-17  Pedro Alves  <palves@redhat.com>

	* target.c (enum terminal_state): New enum.
	(terminal_state): New global.
	(target_terminal_init): New function.
	(target_terminal_inferior): Skip if inferior already owns the
	terminal.
	(target_terminal_ours, target_terminal_ours_for_output): New
	functions.
	* target.h (target_terminal_init): Convert to function prototype.
	(target_terminal_ours_for_output): Convert to function prototype
	and tweak comment.
	(target_terminal_ours): Convert to function prototype and tweak
	comment.
	* windows-nat.c (do_initial_windows_stuff): Call
	target_terminal_init instead of child_terminal_init_with_pgrp.