Add the possibility to load plumed with dlmopen, on systems that support it

[GiovanniBussi]

Currently, runtime loading of plumed kernel uses dlopen.

I would like to add the possibility to use dlmopen, which would allow keeping the whole plumed kernel and also its libraries in a separate linker namespace.

This could facilitate things like:

• the python interpreter calling plumed and a hypothetical python interpreter embedded in plumed (PYCV for PLUMED v2.6 #528) could be separate versions
• plumed and the calling MD code could be linked against different library versions (e.g., boost, or gcc versions)

This is only supported on modern glibc linux. On MacOS, however, if all the libraries are compiled with two-level namespace (which is the default on MacOS) this should already be possible.