Use hash or map in POSIX keys

by Zhongwei Yao for RTEMS Project

The objective of this project is reducing the memory overhead in the implementation of POSIX keys without adding any extra lookup overhead by some proper hash or map algorithm. This feature is required by two reasons: first, the POSIX key area is not properly extended when the number of threads is increased(dynamically created) if POSIX threads are configured as "unlimited", which is a known bug. Second, extra memory is reserved in keys for each thread or task in current implementation, which can be heavy memory overhead when keys increase since key can be dynamically created.