move handing of "set interactive-mode" to gdb_has_a_terminal

move handing of "set interactive-mode" to gdb_has_a_terminal

The real purpose of this setting is really to override what the debugger
would otherwise guess from checking the stdin settings.  So it seems
more natural to see this setting being handled inside gdb_has_a_terminal
rather than input_is_terminal (which checks for other things, such as
whether the input is stdin, for instance).

This patch also adjust the command help and the associated section in
the GDB Manual to be a little clearer about that.

gdb/ChangeLog:

        * inflow.c: Include "gdbcmd.h".
        (interactive_mode): New static global, moved here from top.c.
        (show_interactive_mode): New function, moved here from top.c.
        use gdb_has_a_terminal instead of input_from_terminal_p to
        determine the current mode.
        (gdb_has_a_terminal): Add handling of the "iteractive-mode"
        setting.
        (_initialize_inflow): Add the "set/show interactive-mode"
        commands.  Moved here from top.c, after having adjusted slightly
        the help text.
        * top.c (interactive_mode, show_interactive_mode): Delete, moved
        to inflow.c.
        (input_from_terminal_p): Remove handling of "interactive-mode"
        setting, moved to infow.c.
        (init_main): Remove creation of the "set/show interactive-mode"
        commands, moved to inflow.c.

gdb/doc/ChangeLog:

        * gdb.texinfo (Other Misc Settings): Rework part of the
        documentation of the "set interactive mode" command.