Fix cast warnings when pthread_t is smaller than a pointer

Push conversions from pthread_t to Qt::HANDLE and back into functions.
The casts that were being used didn't work for the unusual 64-bit
pointer/32-bit int combination that QNX is using for 7.0.  HANDLE ends
up as a 64-bit pointer and pthread_t ends up as a 32-bit integer.  g++
considers the precision loss when converting from the 64-bit pointer
to the 32-bit integer an error.  Better to have the casts hidden in
functions so it's easier to adjust them for unusual combinations such
as this.

Change-Id: Ia156b26224a0f7edc1c31e3d1ee8b21191381698
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>