Fixed 'snapshot' rsync command in git-shell

Now 'snapshot' will create archive directory without additional
subdirectory
Added global authorized_keys directory in sshd

- sshd will look for authorized user keys in /etc/ssh/authorized_keys/%u
directory in addition to users own home directories;

- for security reasons users in 'sftponly' group can only login using
  ssh keys from system-wide directory;