rtos: rework rtos_types[] and rtos_try_next()

rtos: rework rtos_types[] and rtos_try_next()

Drop the NULL sentinel at the end of the array and use ARRAY_SIZE()
to bound the loops.
Adapt rtos_try_next() to use ARRAY_SIZE().

While there:
- change to bool the return type of rtos_try_next();
- move rtos_try_next() to avoid the forward declaration.

Change-Id: I1bee11db943b670789e62f1bebe2509bbef451a0
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8855
Tested-by: jenkins