Make {get,set}_inferior_io_terminal inferior methods

This converts the get_inferior_io_terminal and
set_inferior_io_terminal free functions to inferior methods.

Since the related commands are called "tty", "{set,show}
inferior-tty", and MI's "-inferior-tty-{set,show}", to make the
connection between the commands and the code more obvious, the methods
are named set_tty/tty instead of set_io_terminal/io_terminal.

gdb/ChangeLog:

	* fork-child.c (prefork_hook): Adjust.
	* infcmd.c (set_inferior_io_terminal, get_inferior_io_terminal):
	Delete.
	(set_inferior_tty_command, show_inferior_tty_command): Adjust.
	* inferior.c (inferior::set_tty, inferior::tty): New methods.
	* inferior.h (set_inferior_io_terminal, get_inferior_io_terminal):
	Remove declarations.
	(struct inferior) <set_tty, tty>: New methods.
	(struct inferior) <terminal>: Rename to ...
	(struct inferior) <m_terminal>: ... this and make private.
	* main.c (captured_main_1): Adjust.
	* mi/mi-cmd-env.c (mi_cmd_inferior_tty_set): Adjust.
	(mi_cmd_inferior_tty_show): Adjust.
	* nto-procfs.c (nto_procfs_target::create_inferior): Adjust.
	* windows-nat.c (windows_nat_target::create_inferior): Adjust.

gdb/inferior.h

@@ -118,11 +118,6 @@ extern void set_sigint_trap (void);
 
 extern void clear_sigint_trap (void);
 
-/* Set/get file name for default use for standard in/out in the inferior.  */
-
-extern void set_inferior_io_terminal (const char *terminal_name);
-extern const char *get_inferior_io_terminal (void);
-
 /* Collected pid, tid, etc. of the debugged inferior.  When there's
    no inferior, inferior_ptid.pid () will be 0.  */
 
@@ -411,6 +406,14 @@ public:
   inline safe_inf_threads_range threads_safe ()
   { return safe_inf_threads_range (this->thread_list); }
 
+  /* Set/get file name for default use for standard in/out in the
+     inferior.  On Unix systems, we try to make TERMINAL_NAME the
+     inferior's controlling terminal.  If TERMINAL_NAME is nullptr or
+     the empty string, then the inferior inherits GDB's terminal (or
+     GDBserver's if spawning a remote process).  */
+  void set_tty (const char *terminal_name);
+  const char *tty ();
+
   /* Convenient handle (GDB inferior id).  Unique across all
      inferiors.  */
   int num = 0;
@@ -456,9 +459,6 @@ public:
      this inferior.  */
   gdb::unique_xmalloc_ptr<char> cwd;
 
-  /* The name of terminal device to use for I/O.  */
-  gdb::unique_xmalloc_ptr<char> terminal;
-
   /* The terminal state as set by the last target_terminal::terminal_*
      call.  */
   target_terminal_state terminal_state = target_terminal_state::is_ours;
@@ -541,6 +541,9 @@ public:
 private:
   /* The inferior's target stack.  */
   target_stack m_target_stack;
+
+  /* The name of terminal device to use for I/O.  */
+  gdb::unique_xmalloc_ptr<char> m_terminal;
 };
 
 /* Keep a registry of per-inferior data-pointers required by other GDB