prf: add function to retrieve early keying material

This adds a new function gnutls_prf_early to retrieve early keying material. The function shall be used in a handshake hook for GNUTLS_HANDSHAKE_CLIENT_HELLO.

Fixes #329 (closed), #736 (closed).

Checklist

• Code modified for feature
• Test suite updated with functionality tests
• Test suite updated with negative tests
• Documentation updated / NEWS entry present (for non-trivial changes)

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