gdb: don't set breakpoint::pspace in create_breakpoint

I spotted this code within create_breakpoint:

  if ((type_wanted != bp_breakpoint
      && type_wanted != bp_hardware_breakpoint) || thread != -1)
   b->pspace = current_program_space;

this code is only executed when creating a pending breakpoint, and
sets the breakpoint::pspace member variable.

The above code gained the 'thread != -1' clause with this commit:

  commit cc72b2a2
  Date:   Fri Dec 23 17:06:16 2011 +0000

              Introduce gdb.FinishBreakpoint in Python

While the type_wanted checks were added with this commit:

  commit f8eba3c6
  Date:   Tue Dec 6 18:54:43 2011 +0000

      the "ambiguous linespec" series

Before this breakpoint::pspace was set unconditionally.

If we look at how breakpoint::pspace is used today, some breakpoint
types specifically set this field, either in their constructors, or in
a wrapper function that calls the constructor.  So, the watchpoint
type and its sub-class set this variable, as does the catchpoint type,
and all it's sub-classes.

However, code_breakpoint doesn't specifically set this field within
its constructor, though some sub-classes of
code_breakpoint (ada_catchpoint, exception_catchpoint,
internal_breakpoint, and momentary_breakpoint) do set this field.

When I examine all the places that breakpoint::pspace is used, I
believe that in every place where it is expected that this field is
set, the breakpoint type will be one that specifically sets this
field.

Next, I observe two problems with the existing code.

First, the above code is only hit for pending breakpoints, there's no
equivalent code for non-pending breakpoints.  This opens up the
possibility of GDB entering non-consistent states; if a breakpoint is
first created pending and then later gets a location, the pspace field
will be set, while if the breakpoint is immediately non-pending, then
the pspace field will never be set.

Second, if we look at how breakpoint::pspace is used in the function
breakpoint_program_space_exit, we see that when a program space is
removed, any breakpoint with breakpoint::pspace set to the removed
program space, will be deleted.  This makes sense, but does mean we
need to ensure breakpoint::pspace is only set for breakpoints that
apply to a single program space.

So, if I create a pending dprintf breakpoint (type bp_dprintf) then
the breakpoint::pspace variable will be set even though the dprintf is
not really tied to that one program space.  As a result, when the
matching program space is removed the dprintf is incorrectly removed.

Also, if I create a thread specific breakpoint, then, thanks to the
'thread != -1' clause the wrong program space will be stored in
breakpoint::pspace (the current program space is always used, which
might not be the program space that corresponds to the selected
thread), as a result, the thread specific breakpoint will be deleted
when the matching program space is removed.

If we look at commit cc72b2a2 which added the 'thread != -1'
clause, we can see this change was entirely redundant, the
breakpoint::pspace is also set in bpfinishpy_init after
create_breakpoint has been called.  As such, I think we can safely
drop the 'thread != -1' clause.

For the other problems, I'm proposing to be pretty aggressive - I'd
like to drop the breakpoint::pspace assignment completely from
create_breakpoint.  Having looked at how this variable is used, I
believe that it is already set elsewhere in all the cases that it is
needed.  Maybe this code was needed at one time, but I can't see how
it's needed any more.

There's tests to expose the issues I've spotted with this code, and
there's no regressions in testing.