fips140: aligned code with documentation

That is, introduce the documented but unimplemented macros GNUTLS_FIPS140_SET_RELAX_MODE() and GNUTLS_FIPS140_SET_STRICT_MODE().

Checklist

• Code modified for feature
• Test suite updated with functionality tests
• Documentation updated

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTION.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code