security/letsencrypt.sh: Update to 2016-02-12

• Update to 2016-02-16
• Add options for ZSH
• Change periodic script to weekly
  • Add unprivileged renewal option
  • Add privileged post-script option

PR: 206976
Reviewed by: feld (mentor), koobs (mentor), sascha (maintainer)
Approved by: sascha (maintainer), feld (mentor)
Differential Revision: D5264