libssh: do not let libssh create socket

By default, libssh creates a new socket, instead of using the socket
created by curl for SSH connections.

Pass the socket created by curl to libssh using ssh_options_set() with
SSH_OPTIONS_FD directly after ssh_new(). So libssh uses our socket
instead of creating a new one.

This approach is very similar to what is done in the libssh2 code, where
the socket created by curl is passed to libssh2 when
libssh2_session_startup() is called.

Fixes #3491
Closes #3495