Replace trivalue options with plain booleans

Replace trivalue options with plain booleans

Our CMake auto-detection has been a long discussion since
it both complicates the CMake code and leads to complex
interactions when multiple auto-settings interact with
each other, and when/how things can change.

This is a first step towards removing such auto settings,
by altering "nice to have, but not required" to "off" by
default (GMX_BUILD_HELP, GMX_HWLOC).

The option GMX_LOAD_PLUGINS was on by default, and
the build generates errors if it's turned off, so for now
I set it to "on", and will adress the compile error when
it's disabled in a separate change.

With these changes, the trivalue options are no more.