scripts: west: fix some intel_s1000 runner issues

There are a few issues with the intel_s1000 runner:

- it doesn't attach to the client when running debug (this is
  a part of the command's contract specified in core.py)

- it uses hard-coded sleeps when running subprocesses instead of
  waiting for them to terminate

- it re-implements pre-existing popen_ignore_int() functionality (in a
  way that is not portable to Windows) rather than using it directly

Fix these issues.

Signed-off-by: Marti Bolivar <marti@opensourcefoundries.com>