(stable) fix dashed ssh hostname issue CVE-2017-12976

Security fix: Disallow hostname starting with a dash, which would get
passed to ssh and be treated an option. This could be used by an attacker
who provides a crafted ssh url (for eg a git remote) to execute arbitrary
code via ssh -oProxyCommand.

The same class of security hole recently affected git itself,
CVE-2017-1000117.

Method: Identified all places where ssh is run, by git grep '"ssh"'
Converted them all to use a SshHost, if they did not already, for
specifying the hostname.

SshHost was made a data type with a smart constructor, which rejects
hostnames starting with '-'.

Note that git-annex already contains extensive use of Utility.SafeCommand,
which fixes a similar class of problem where a filename starting with a
dash gets passed to a program which treats it as an option.

This was backported by Antoine Beaupré, from the upstream stable patch
provided by Joey Hess.