rtos: Rewrite rtos_qsymbol() and fix auto-detect false positive

Matthias Blaicher submitted a patch at http://openocd.zylin.com/#/c/891/
to fix the false positive; when no RTOS was detected OpenOCD used the
last RTOS in the list.

While reviewing the code affected by Matthias' patch a rewrite seemed
appropriate, to make the code readable.

Matthias has abandoned his change and this change also fixes the false
positive.

Change-Id: Ic3327ccd036da52ba0a7e21ef93018205e74149c
Signed-off-by: Peter Stuge <peter@stuge.se>
Reviewed-on: http://openocd.zylin.com/895
Reviewed-by: Matthias Blaicher <matthias@blaicher.com>
Tested-by: jenkins