Adding GLOBAL keyword to LOAD action

[Iximiel]

Description

Knowing the problems with the new direction of pycv in Unix I added the possibility to LOAD shared object with the RTLD_GLOBAL option, if needed the standard behaviour is not changed.

I changed some names in the load method of PlumedMain, for readability.

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: 3 lines in your changes are missing coverage. Please review.

Comparison is base (ae54bc5) 84.11% compared to head (7a9de3d) 84.11%.

:exclamation: Current head 7a9de3d differs from pull request most recent head 05ac10b. Consider uploading reports for the commit 05ac10b to get more accurate results

Files	Patch %	Lines
src/core/PlumedMain.cpp	87.50%	2 Missing :warning:
src/tools/DLLoader.cpp	80.00%	1 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #985 +/- ## ========================================== - Coverage 84.11% 84.11% -0.01% ========================================== Files 602 602 Lines 56083 56092 +9 ========================================== + Hits 47176 47183 +7 - Misses 8907 8909 +2 ```

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