R1:3dea92081bbe

Fix an issue where passphrase-protected private keys were stored without discarding passphrases

Summary:
Ref T13454. See <https://discourse.phabricator-community.org/t/newly-created-ssh-private-keys-with-passphrase-not-working-anymore/3883>.

After changes to distinguish between invalid and passphrase-protected keys, SSH private key management code incorrectly uses "-y ..." ("print public key") when it means "-p ..." ("modify input file, removing passphrase"). This results in the command having no effect…