Should push_target pop all targets?

[Andrew Cagney]

> On GNU/Linux, attach breaks (yet the exact same test passes on BSD ...)

I believe this happens:

(gdb) attach pid
-> calls attach_command()
    -> calls ... find_default_run_target's attach
    -> calls exec_file_attach()
       -> calls push_target (&exec_ops)
          pops the "run" target

outch!

Andrew