libssh2: 1.6.0 -> 1.7.0

    This release includes the following changes:
      o libssh2_session_set_last_error: Add function
      o mac: Add support for HMAC-SHA-256 and HMAC-SHA-512
      o WinCNG: support for SHA256/512 HMAC
      o kex: Added diffie-hellman-group-exchange-sha256 support
      o OS/400 crypto library QC3 support
    This release includes the following security advisory:
      o diffie_hellman_sha256: convert bytes to bits
        CVE-2016-0787: http://www.libssh2.org/adv_20160223.html
    This release includes the following bugfixes:
      o SFTP: Increase speed and datasize in SFTP read
      o openssl: make libssh2_sha1 return error code
      o openssl: fix memleak in _libssh2_dsa_sha1_verify()
      o cmake: include CMake files in the release tarballs
      o Fix builds with Visual Studio 2015
      o hostkey.c: Fix compiling error when OPENSSL_NO_MD5 is defined
      o GNUmakefile: add support for LIBSSH2_LDFLAG_EXTRAS
      o GNUmakefile: add -m64 CFLAGS when targeting mingw64
      o kex: free server host key before allocating it (again)
      o SCP: add libssh2_scp_recv2 to support large (> 2GB) files on windows
      o channel: Detect bad usage of libssh2_channel_process_startup
      o userauth: Fix off by one error when reading public key file
      o kex: removed dupe entry from libssh2_kex_methods
      o _libssh2_error: Support allocating the error message
      o hostkey: fix invalid memory access if libssh2_dsa_new fails
      o hostkey: align code path of ssh_rsa_init to ssh_dss_init
      o libssh2.pc.in: fix the output of pkg-config --libs
      o wincng: fixed possible memory leak in _libssh2_wincng_hash
      o wincng: fixed _libssh2_wincng_hash_final return value
      o add OpenSSL 1.1.0-pre2 compatibility
      o agent_disconnect_unix: unset the agent fd after closing it
      o sftp: stop reading when buffer is full
      o sftp: Send at least one read request before reading
      o sftp: Don't return EAGAIN if data was written to buffer
      o sftp: Check read packet file offset
      o configure: build "silent" if possible
      o openssl: add OpenSSL 1.1.0-pre3-dev compatibility
      o GNUmakefile: list system libs after user libs