Request the Publickey subsystem from an already connected SSH2 server.
The publickey subsystem allows an already connected and authenticated
client to manage the list of authorized public keys stored on the
target server in an implementation agnostic manner.
If the remote server does not support the publickey subsystem,
the ssh2_publickey_init() function will return
SSH2 Publickey Subsystem resource for use
with all other ssh2_publickey_*() methods, or
FALSE on failure.
The publickey subsystem is used for managing publickeys on a server to which the client is already authenticated. To authenticate to a remote system using publickey authentication, use the ssh2_auth_pubkey_file() function instead.