rtos: Fix regression preventing use of first RTOS & clean up rtos_qsymbol()

ThreadX support was not working due to it being first in the list of RTOS - regression.
Auto-detect off, an RTOS was always be marked as successfully detected, even if symbols are not found.
Lines 223-227 were unnecessary as they are done in rtos_try_next()
Added lots of comments
Improved readability by separating: GDB not finding a symbol vs no more symbols being available

Regression caused by patch which was allowed only 52 minutes for review : http://openocd.zylin.com/895

Change-Id: Ib4decb01db595ddb3796837c6d8338ce6b9a91ca
Signed-off-by: Evan Hunter <ehunter@broadcom.com>
Reviewed-on: http://openocd.zylin.com/986
Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>