Improve handling of host buffers
Improve handling of host buffers

xgpuInit() now calls xgpuSetHostInputBuffer() and
xgpuSetHostOutputBuffer() to register/allocate host buffers, thereby
eliminating some code duplication.

The length passed to cudaHostRegister in xgpuSetHostInputBuffer() and
xgpuSetHostOutputBuffer() is now rounded up to the next multiple of
PAGE_SIZE, which is determiend at runtime via a call to sysconf().

GPU-correlators/xGPU,GPU-correlators/xGPU,GPU-correlators/xGPU