shell: Fix GUI build

* Fix shell_exit() signature, it's called without any argument.
native: Use stderr for logging

Since we will hopefully one day use stdout as a CTK display,
use stderr instead for log output, which can be redirected to a file.
tapdev: Log to stderr; add tracing

Use stderr for logging to avoid polluting stdout.
Add some DEBUG tracing.
native: Fix trailing whitespace
Merge pull request #180 from mmuman/cleanup-and-fixes

Cleanup and fixes
Merge branch 'master' of git://github.com/contiki-os/contiki into experiments