rtos: Rewrite rtos_create() for readability

rtos: Rewrite rtos_create() for readability

The new code is almost functionally equivalent to the old; besides
error handling the only difference is that the code is now readable.

Many thanks to Matthias Blaicher for pointing out an iteration error
in the rtos_try_next() change, which also affected this change.

Change-Id: If38b87439e9de2303b220b3a7e3200ceaa8391da
Signed-off-by: Peter Stuge <peter@stuge.se>
Reviewed-on: http://openocd.zylin.com/893
Tested-by: jenkins
Reviewed-by: Matthias Blaicher <matthias@blaicher.com>