gnutls_init: added flag for automatic re-authentication

This introduces the GNUTLS_AUTO_REAUTH gnutls_init() flag and makes re-authentication under TLS simpler to enable and use.

Checklist

• Code modified for feature
• Test suite updated with functionality tests
• Documentation updated
• Verify that new code is working in non-blocking mode

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