Do not send queries on secondary UIs

Do not send queries on secondary UIs

This is a follow-up to

  https://sourceware.org/ml/gdb-patches/2017-02/msg00261.html

This patch restricts queries to the main UI, which allows to avoid two
different problems.

The first one is that GDB is issuing queries on secondary MI channels
for which a TTY is allocated.  The second one is that GDB is not able to
handle queries on two (CLI) UIs simultaneously.  Restricting queries to
the main UI allows to bypass these two problems.

More details on how/why these two problems happen:

1. Queries on secondary MI UI

  The current criterion to decide if we should query the user is whether
  the input stream is a TTY.  The original way to start GDB in MI mode
  from a front-end was to create a subprocess with pipes to its
  stdin/stdout.  In this case, the input was considered non-interactive
  and queries were auto-answered.  Now that front-ends can create the MI
  channel as a separate UI connected to a dedicated TTY, GDB now
  considers this input stream as interactive and sends queries to it.
  By restricting queries to the main UI, we make sure we never query on
  the secondary MI UI.

2. Simultaneous queries

  As Pedro stated it, when you have two queries on two different CLI UIs
  at the same time, you end up with the following pseudo stack:

  #0 gdb_readline_wrapper
  #1 defaulted_query                 // for UI #2
  #2 handle_command
  #3 execute_command ("handle SIGTRAP" ....
  #4 stdin_event_handler             // input on UI #2
  #5 gdb_do_one_event
  #7 gdb_readline_wrapper
  #8 defaulted_query                 // for UI #1
  #9 handle_command
  #10 execute_command ("handle SIGINT" ....
  #11 stdin_event_handler            // input on UI #1
  #12 gdb_do_one_event
  #13 gdb_readline_wrapper

  trying to answer the query on UI #1 will therefore answer for UI #2.

  By restricting the queries to the main UI, we ensure that there will
  never be more than one pending query, since you can't have two queries
  on a UI at the same time.

I added a snippet to gdb.base/new-ui.exp to verify that we get a query
on the main UI, but that we don't on the secondary one (or, more
precisely, that it gets auto-answered).

gdb/ChangeLog:

	* utils.c (defaulted_query): Don't query on secondary UIs.

gdb/testsuite/ChangeLog:

	* gdb.base/new-ui.exp (do_test): Test queries behavior on main
	and extra UIs.