The KDF API is a bit messy to use, a few changes that would make it more ergonomic:

• All kdfs with a variable output size should permit the EVP_KDF_CTRL_SET_MAC_SIZE control
• Proliferation of "info" controls: HKDF should support SET_INFO. kdfs should either all support RESET_INFO and ADD_INFO, or none of them should
• The SSH KDF's XCGHASH and SESSION_ID sound a lot like salt/info. Could they reuse the same options?