Prior to this commit, if the ssh-config command was invoked within
cygwin or msys2, it would show a regular windows style path for private
keys rather than a path that could be used within msys2 or cygwin. This
commit updates that behavior by converting all of the private key paths
to the proper msys2 or cygwin path if the platform is windows and the
command was invoked from one of those two shells.
since we merged multiple private keys, the :private_key_path item of
ssh_info now comes back as an array. the ssh-config command had not yet
been updated to handle this properly.
here we fix that oversight and add a few unit tests around the config
generation.