Make usleep() overridable by the thread libraries so they can provide cancellation points.
Make usleep() overridable by the thread libraries so they can provide
cancellation points.

Noticed by:	phk

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase