libssh2 1.8.0

This release includes the following changes:

 o added a basic dockerised test suite
 o crypto: add support for the mbedTLS backend

This release includes the following bugfixes:

 o libgcrypt: fixed a NULL pointer dereference on OOM
 o VMS: can't use %zd for off_t format
 o VMS: update vms/libssh2_config.h
 o windows: link with crypt32.lib
 o libssh2_channel_open: speeling error fixed in channel error message
 o msvc: fixed 14 compilation warnings
 o tests: HAVE_NETINET_IN_H was not defined correctly
 o openssl: add OpenSSL 1.1.0 compatibility
 o cmake: Add CLEAR_MEMORY option, analogously to that for autoconf
 o configure: make the --with-* options override the OpenSSL default
 o libssh2_wait_socket: set err_msg on errors
 o libssh2_wait_socket: Fix comparison with api_timeout to use milliseconds
 
This release would not have looked like this without help, code, reports and
advice from friends like these:

  Alexander Lamaison, Antenore Gatta, Brad Harder, Charles Collicutt,
  Craig A. Berry, Dan Fandrich, Daniel Stenberg, Kamil Dudka, Keno Fischer,
  Taylor Holberton, Viktor Szakats, Will Cosgrove, Zenju
  (12 contributors)

        Thanks! (and sorry if I forgot to mention someone)
