We used to hide current WineDbg instance when displaying processes' list
(command 'info proc'). This can potentially generate some "dangling"
processes in the hierarchy (related to this WineDbg instance):
- conhost.exe
- start.exe (when launched from unix shell without full path
  to winedbg.exe)

Also, print a more comprehensive error message when trying to attach to
itself (now that debugger's PID is more easily available).
Signed-off-by: Eric Pouech <eric.pouech@gmail.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>