include/ssh: Simplify SSH module API

include/ssh: Simplify SSH module API

The ssh_tunnel_close() and ssh_proxy_close() functions do essentially
the same thing, so they should be merged into one. Further, both of
the functions require the user to remember multiple values from the
ssh_*_open() call in order to close the connection. This seems
unnecessarily complicated.
This commit adds the ssh_close() function as a replacement for
ssh_tunnel_close() and ssh_proxy_close(). Further, the ssh_*_open()
functions now return a handle that can be passed to ssh_close() to
close the respective connection.