make SSH2.xs build under non-threaded perls

- swap order of my_cxt_t/START_MY_CXT
- #define my_perl away under non-threaded perls
- make non-exported symbols static
- define a dummy openssl_threads_init() under non-threaded perls
Merge pull request #13 from mauke/fix-unthreaded

make SSH2.xs build under non-threaded perls