kernel: restrict user threads to worsen priority
kernel: restrict user threads to worsen priority

User threads aren't trusted and shouldn't be able to alter the
scheduling assumptions of the system by making thread priorities more
favorable.

Signed-off-by: Andrew Boie <aab045b28152628be6af0e556050d0381fd0a34d@intel.com>

ldts/zephyr,finikorg/zephyr,explora26/zephyr,kraj/zephyr,galak/zephyr,mbolivar/zephyr,Vudentz/zephyr,kraj/zephyr,nashif/zephyr,galak/zephyr,zephyriot/zephyr,ldts/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr,mbolivar/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr,finikorg/zephyr,kraj/zephyr,zephyriot/zephyr,ldts/zephyr,fbsder/zephyr,mbolivar/zephyr,explora26/zephyr,fbsder/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,aceofall/zephyr-iotos,mbolivar/zephyr,kraj/zephyr,ldts/zephyr,ldts/zephyr,zephyriot/zephyr,explora26/zephyr,explora26/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,punitvara/zephyr,finikorg/zephyr,mbolivar/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,fbsder/zephyr,galak/zephyr,punitvara/zephyr,galak/zephyr,zephyriot/zephyr,zephyriot/zephyr,punitvara/zephyr,aceofall/zephyr-iotos,punitvara/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,fbsder/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,fbsder/zephyr,aceofall/zephyr-iotos,GiulianoFranchetto/zephyr,aceofall/zephyr-iotos,Vudentz/zephyr,aceofall/zephyr-iotos,zephyrproject-rtos/zephyr