( resource session )
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 FALSE.
Note: 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()