Add support for SHA2 HMAC algorithms

This commit adds support for SHA2 HMAC algorithms for signing packets.
Add options support for HMAC
Add new options last for ABI compatibility
Move updating output so we don't potentially leak

If ssh_mac_ctx_init would fail there was a potential memory leak here,
since the *output pointer wouldn't have been updated in that case.
session: getters for current HMAC algorithms