patch: consistent passphrase prompt wording

[Jeroen Ooms <jeroen@xxxxxxxxxxxx>]

The prompt wording for requesting the passphrase for openssh keys is
just "Passphrase". For other implementations it is: "Passphrase for
private key:" which makes it more obvious to the user which password
is needed.

Patch: https://patch-diff.githubusercontent.com/raw/jeroen/libssh/pull/1.patch