RFR: 8235908: omit ThreadPriorityPolicy warning when value is set from image

[Matthias Neugschwandtner]

Hello everyone,

Please review this change, which omits the warning that 
ThreadPriorityPolicy=1 requires system level permission if the value is 
set via jlink, the same way it is omitted if the value is set by default.

https://bugs.openjdk.java.net/browse/JDK-8235908
http://cr.openjdk.java.net/~mneugschwand/8235908

Thank you,
Matthias