Make it easier to attach the remote debugger.

Make it easier to attach the remote debugger.

base/remote_gdb.cc:
    Keep track of a vector of all of the usable debuggers and provide
    a current_debugger variable that can be set from gdb to specify
    which remote debugger is desired.
    If debugger() is called and the current_debugger is not attached,
    call accept on its listener so we can still attach a remote debugger
    from the gdb prompt if we had forgotten to before.
    Print out more information when the simulator starts so we can
    distinguish debuggers more easily.
base/remote_gdb.hh:
    Have a remote debugger instance keep track of its listener.
    Number the remote debuggers so it's easier to figure out which is which.

--HG--
extra : convert_revision : 97119597ac3772ea4df6da3b3a4827f50253a51f