Log Message:
Updated libssh2 to version 1.6.0.

Changelog:

Changes:

    Added libssh2_userauth_publickey_frommemory()

Bug fixes:

    wait_socket: wrong use of difftime()
    userauth: Fixed prompt text no longer being copied to the prompts struct
    mingw build: allow to pass custom CFLAGS
    Let mansyntax.sh work regardless of where it is called from
    Init HMAC_CTX before using it
    direct_tcpip: Fixed channel write
    WinCNG: fixed backend breakage
    OpenSSL: caused by introducing libssh2_hmac_ctx_init
    userauth.c: fix possible dereferences of a null pointer
    wincng: Added explicit clear memory feature to WinCNG backend
    openssl.c: fix possible segfault in case EVP_DigestInit fails
    wincng: fix return code of libssh2_md5_init()
    kex: do not ignore failure of libssh2_sha1_init()
    scp: fix that scp_send may transmit not initialised memory
    scp.c: improved command length calculation
    nonblocking examples: fix warning about unused tvdiff on Mac OS X
    configure: make clear-memory default but WARN if backend unsupported
    OpenSSL: Enable use of OpenSSL that doesn't have DSA
    OpenSSL: Use correct no-blowfish #define
    kex: fix libgcrypt memory leaks of bignum
    libssh2_channel_open: more detailed error message
    wincng: fixed memleak in (block) cipher destructor