gdb: add the 'set/show suppress-cli-notifications' command

GDB already has a flag to suppress printing notification events, such
as thread and inferior context switches, on the CLI.  This is used
internally when executing commands.  Make the flag available to the
user via a new command.  This is expected to be useful in scripts.

For instance, suppose that when Inferior 1 gets to a certain state,
you want to add and set up a new inferior using the commands below,
but you also want to have a reduced/clean output.

  define do-setup
    printf "Setting up Inferior 2...\n"
    add-inferior -exec a.out
    inferior 2
    break file.c:3
    run
    inferior 1
    printf "Done\n"
  end

Currently, GDB prints

  (gdb) do-setup
  Setting up Inferior 2...
  [New inferior 2]
  Added inferior 2 on connection 1 (native)
  [Switching to inferior 2 [<null>] (/tmp/a.out)]
  Breakpoint 2 at 0x1155: file file.c, line 3.

  Thread 2.1 "a.out" hit Breakpoint 2, main () at file.c:3
  3         return 0;
  [Switching to inferior 1 [process 7670] (/tmp/test)]
  [Switching to thread 1.1 (process 7670)]
  #0  main () at test.c:2
  2         int a = 1;
  Done

GDB's Python API make it possible to capture and return GDB's output,
but this does not work for all the streams.  In particular, CLI
notification events are not captured:

  (gdb) python gdb.execute("do-setup", False, True)
  [Switching to inferior 2 [<null>] (/tmp/a.out)]

  Thread 2.1 "a.out" hit Breakpoint 2, main () at file.c:3
  3         return 0;
  [Switching to inferior 1 [process 8263] (/tmp/test)]
  [Switching to thread 1.1 (process 8263)]
  #0  main () at test.c:2
  2         int a = 1;

You can use the new "set suppress-cli-notifications" command to
suppress the output:

  (gdb) set suppress-cli-notifications on
  (gdb) do-setup
  Setting up Inferior 2...
  [New inferior 2]
  Added inferior 2 on connection 1 (native)
  Breakpoint 2 at 0x1155: file file.c, line 3.
  Done

1 files changed, 8 insertions, 0 deletions

diff --git a/gdb/NEWS b/gdb/NEWS
index 8da68c8..c739930 100644
--- a/gdb/NEWS
+++ b/gdb/NEWS
@@ -81,6 +81,14 @@ maint show gnu-source-highlight enabled
   styling to a particular source file, then the Python Pygments
   library will be used instead.
 
+set suppress-cli-notifications (on|off)
+show suppress-cli-notifications
+  This controls whether printing the notifications is suppressed for CLI.
+  CLI notifications occur when you change the selected context
+  (i.e., the current inferior, thread and/or the frame), or when
+  the program being debugged stops (e.g., because of hitting a
+  breakpoint, completing source-stepping, an interrupt, etc.).
+
 * Changed commands
 
 maint packet