Add support for SSH_OPTIONS_PROXYJUMP

[fmang]

To make life easier in the future I have added a low-level binding to
ssh_options_set and a high-level wrapper class for Session in Ruby.

I haved merged the C initialize method into the allocator and defined a
new Ruby initialize method. Though evil, Session.allocate.connect
should not make the whole Ruby process crash like it does now.

Without authentication the test is not convincing yet, but I could
confirm that session.add_proxy_jump "localhost" establishes a
connection to my host’s SSH server using my user’s default keys.

Calling ssh_options_set every time a new proxy jump is added is a bit
wasteful but multiple jumps are rare and the intended API is:

session.add_proxy_jump "foo", authenticate: -> (jump_session) { … }
session.add_proxy_jump "bar", authenticate: -> (jump_session) { … }

Rather than the counter-intuitive libssh way:

session.proxy_jumps = ["foo", "bar"]
session.append_proxy_jump_callback authenticate: -> (jump_session) { … }
session.append_proxy_jump_callback authenticate: -> (jump_session) { … }

-Wno-unused-parameter silences warnings in Ruby’s headers, which used to
cause noise when building.