[Security]Check file permissions of system wide ssh_config file.

[bagajjal]

Windows don't have a system-wide ssh folder ($env:Programdata\ssh) by default.
Assume a non-admin user creates the system-wide ssh folder and then the admin user installs the win32-OpenSSH.
Admin user later manually creates the system-wide ssh_config ($env:Programdata\ssh\ssh_config). By default, this file inherits the parent folder permissions i.e., the non-admin user has the write permission.
This is not desirable.

Proposed fix - ssh.exe should check the file permissions on system-wide ssh_config file. If this file has write permissions for non admin users then fail the ssh connection.

[bagajjal]

Associated upstream bug,
https://bugzilla.mindrot.org/show_bug.cgi?id=3277

OpenBSD team plan to fix this in next version i.e., OpenSSH v8.6