support compiling against old libssh2 versions

Add guards for Net::SSH2 known_hosts method and the full
Net::SSH2::KnownHosts class.

Ignore "Compress" option on "connect" method when
"libssh2_session_flag" is not available.

Fix error on "libssh2_session_flag" not-available wrapper.
regenerate constant wrappers
fix harmless typo