Fix to work with the latest CVS version of libssh2

* As of (LIBSSH2_APINO >= 200706012030) there are not *nb() functions
* As of (LIBSSH2_APINO >= 200706012030) most libssh2_*() functions
  can return LIBSSH2_ERROR_EAGAIN to indicate that the call would block.

To make the code work as previously, blocking, all the code has been
updated so that when (LIBSSH2_APINO >= 200706012030) it loops simulating
blocking.  This allows the existing code to function and not hold up
the upcoming release.