Add support for ECDSA key exchange and SHA256 MAC.
Note these patches were cherry-piecked from the following upstream commits:
https://github.com/libssh2/libssh2/commit/bbc43cb
https://github.com/libssh2/libssh2/commit/aba34f5
https://github.com/libssh2/libssh2/commit/62b825c
Submitted by: Andrew Heybey <ath@heybey.org>
Reviewed by: sbz (maintainer)