Fix some OpenCL issues

Fix some OpenCL issues

Added routine to convert error codes into more helpful
diagnostics. Called it in one place that needed some troubleshooting,
but an overhaul of OpenCL error handling is needed (in master branch).

Introduced new OpenCL control variable to indicate when there is a
non-local event upon which it is valid to wait, since it is an error
to wait upon an ocl_event that was never returned by an API call.

Converted mdrun integration tests to rely on the improved automated
resource assignment in 5.1, because that copes better with the
limitations of the OpenCL implementation.

Worked around limitation where real MPI + OpenCL can't use more than
one GPU on a node, by disabling GPU support for that test case.

Fixed inappropriate use of mdrun -nt, where the number of thread-MPI
ranks was intended.

Updated install guide.

Fixes #1871

Change-Id: I11e6b2bdb6f7f91489f3ec0d671081d99661fa62