make signed parameters more useful:
- added secret for parameter signature (can be configured
in config file in section "ns/server/${server}/acs"
parameter "parametersecret"
- allow specification of expiration for signed parameters via
"max_age" in export_vars (eg. "user_id:sign(max_age=20)", meaning:
valid for 20 seconds)
- don't advertise deprected procs in documentation