openssh: security bump to version 7.6p1

openssh: security bump to version 7.6p1

Fixes CVE-2017-15906 - The process_open function in sftp-server.c in OpenSSH
before 7.6 does not properly prevent write operations in readonly mode,
which allows attackers to create zero-length files.

For more details, see the release notes:
https://www.openssh.com/txt/release-7.6

Also add a hash for the license file while we're at it.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>