acs-admin-procs.tcl

check_expired_certificates: automated certificate nenewal for letsencrypt

This change reduce maintenance effort by automating certificate
renewal. When the NaviServer letsencrypt module is installed and
configured, the background operation check_expired_certificates will
automatically update the certificates when these expire soon (as
defined by the "ExpireCertificateWarningPeriod" parameter of
acs-admin). When a recent version of NaviServer is used that supports
certificate refetch on SIGHUP, the new certificates are automatically
updated without a server restart.

Prerequisites:
- Recent version of letsencrypt NaviServer module installed (0.6)
and configured
- Recent version of NaviServer (currently Bitbucket tip) for automated
certificate reloading

When the recent letsencrypt module is not installed,
check_expired_certificates sends expiration warnings as usual.
Therefore, it is also useful for sites using certificates from
different sources.

This new functionality was used for latest certificate renewal on
openacs.org.