Adding a RAII class to promote an object containt a symbol to be dlopened with RTLD_GLOBAL

[Iximiel]

Description

As we were discussing, I modified @GiovanniBussi's class to fit into plumed, with warnings.

I also reverted the changes from #985, since DLLoader::EnsureGlobalDLOpen makes using RTLD_GLOBAL more granular.

For example, on linux, an embedded python interpreter needs its .so to be loaded with RTLD_GLOBAL, for importing compiled modules, with this you can make only libPython*.so GLOBAL, and not the other part of the code that rely on it.

I am using DLLoader as a namespace for EnsureGlobalDLOpen.

Target release

I would like my code to appear in release 2.10

Type of contribution

• [X] changes to code or doc authored by PLUMED developers, or additions of code in the core or within the default modules
• [ ] changes to a module not authored by you
• [ ] new module contribution or edit of a module authored by you

Copyright

• [X] I agree to transfer the copyright of the code I have written to the PLUMED developers or to the author of the code I am modifying.

• [ ] the module I added or modified contains a COPYRIGHT file with the correct license information. Code should be released under an open source license. I also used the command cd src && ./header.sh mymodulename in order to make sure the headers of the module are correct.

Tests

• [ ] I added a new regtest or modified an existing regtest to validate my changes.
• [ ] I verified that all regtests are passed successfully on GitHub Actions.

[codecov-commenter]

Codecov Report

Attention: 8 lines in your changes are missing coverage. Please review.

Comparison is base (91d56cb) 84.10% compared to head (b81cac6) 84.06%.

:exclamation: Current head b81cac6 differs from pull request most recent head 7d301b4. Consider uploading reports for the commit 7d301b4 to get more accurate results

Files	Patch %	Lines
src/tools/DLLoader.cpp	20.00%	8 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## autoconf-dladdr #1000 +/- ## =================================================== - Coverage 84.10% 84.06% -0.04% =================================================== Files 602 602 Lines 56231 56233 +2 =================================================== - Hits 47292 47272 -20 - Misses 8939 8961 +22 ```

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