Use two-level namespace link on osx

Now that libplumedKerneldylib does not depend anymore on libplumed.dylib
it is possible to use two-level namespace on osx.

This change has two implications:
- the shared library used to resolve symbols during linking is recorded
  and used at runtime.
- all symbols should be available at link time

A side effect of this is that while compiling dylibs for the LOAD
action we should link them to the libplumedKernel as well.
For consistency I do the same on Linux, though it is not necessary in principle.