Support ecdsa-sha2-nistp256 SSH keys

Summary:
See discussion in <https://github.com/facebook/phabricator/issues/430>.

(If we end up with more than like 5 of these we should probably make this a warning or something instead, the only goal is to prevent user error.)

Test Plan:
{F79196}

{F79197}

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran, enko

Differential Revision: https://secure.phabricator.com/D7522