expose kex and MAC algos as public constants

golang/crypto is inconsistent with regards to which string constants are exported as part of the public API. This should be a fairly uncontroversial change if we wish to upstream because KeyAlgo and CertAlgo constants are already exported (while KexAlgo and MacAlgo are not).

This MR is by no means comprehensive as there are plenty of other ciphers and protocol-related constants that remain private, it is just the minimum change necessary to replace hardcoded strings in gitlab-sshd server config, for example.