* top.c: Include interps.h.
(execute_command): If the target can async, but the interpreter is
in sync mode, synchronously wait for the command to finish before
returning.
(execute_command_to_string): Force the interpreter to sync mode.
* infrun.c: Include interps.h.
(fetch_inferior_event): Don't restore the prompt yet if the
interpreter is in sync mode.
* interps.c (interpreter_async): New global.
* interps.h (interpreter_async): Declare.
* inf-loop.c: Include interps.h.
(inferior_event_handler): Don't print the language change or run
breakpoint commands yet if the interpreter in is sync mode.
* main.c (captured_command_loop): Flip the interpreter to async
mode.
* cli/cli-script.c: Include interps.h.
(execute_user_command, while_command, if_command): Force the
interpreter to sync mode.
* python/python.c: Include interps.h.
(python_command, execute_gdb_command): Force the interpreter to
sync mode.