Non-stop mode requires that breakpoints always be inserted
in the inferior.  We currently set that with "set breakpoints always-inserted"
prior to switching to non-stop mode with "set non-stop on".

The default setting of "set breakpoints always-inserted" is "off",
as that is how GDB has been behaving for ages.

Since non-stop requires breakpoints always-in, its just cumbersome
to have to issue more than one command to enable non-stop mode.

So, this patch changes the "set breakpoints always-inserted" setting to
be a three-state.  on and off, the same as before, and a new mode,
"follow-non-stop".  In the latter mode, GDB will behave as "on", if
we're in non-stop mode, and as "off" if we're in all-stop mode.

This mode will be the default.

 (gdb) show non-stop
 Controlling the inferior in non-stop mode is off.

We're in all-stop.

 (gdb) show breakpoint always-inserted
 Always inserted breakpoint mode is follow-non-stop (currently off).

GDB shows that the effect is as if "off".

 (gdb) set non-stop on

Turns non-stop on.

 (gdb) show breakpoint always-inserted
 Always inserted breakpoint mode is follow-non-stop (currently on).

GDB shows that the effect is as if "on".

 (gdb) set non-stop off

Back to all-stop.

 (gdb) set breakpoint always-inserted on

Force "on".  Useful for testing.

 (gdb) show breakpoint always-inserted
 Always inserted breakpoint mode is on.

Now GDB shows that "on", independently of the non-stop mode.

What do you think?

If the idea is sound, does the patch look ok?
How about the docs?

-- 
Pedro Alves