libssh2: fix variable type

This led to a conversion warning on 64-bit MinGW, which has 32-bit
`long` but 64-bit `size_t`.

Closes https://github.com/curl/curl/pull/4823

misc: Copyright year out of date, should be 2020

Follow-up to recent commits

[skip ci]

libssh2: add support for forcing a hostkey type

- Allow forcing the host's key type found in the known_hosts file.

Currently, curl (with libssh2) does not take keys from your known_

libssh2: add support for forcing a hostkey type

- Allow forcing the host's key type found in the known_hosts file.

Currently, curl (with libssh2) does not take keys from your known_hosts
file into account when talking to a server. With this patch the
known_hosts file will be searched for an entry matching the hostname
and, if found, libssh2 will be told to claim this key type from the
server.

Closes https://github.com/curl/curl/pull/4747

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libssh2: add support for ECDSA and ed25519 knownhost keys

... if a new enough libssh2 version is present.

Source: https://curl.haxx.se/mail/archive-2019-12/0023.html
Co-Authored

libssh2: add support for ECDSA and ed25519 knownhost keys

... if a new enough libssh2 version is present.

Source: https://curl.haxx.se/mail/archive-2019-12/0023.html
Co-Authored-by: Daniel Stenberg
Closes #4714

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lib: silence conversion warnings

Closes https://github.com/curl/curl/pull/4444

libssh2: part of conditional expression is always true: !result

Fixes warning detected by PVS-Studio
Fixes #4374

ssh: add a generic Curl_ssh_version function for SSH backends

Closes #4235

vssh: move ssh init/cleanup functions into backend code

vssh: create directory for SSH backend code