Re-arrange batch system front-end to match Web UI behavior

* autoswap time and idleswap time defaults are inherited from the
  associated sitevars.

 - "general/autoswap_threshold", and "idle/threshold", respectively

* autoswap default state follows associated sitevar ("general/autoswap_mode").

* Any user-requested autoswap duration is checked against the
  "general/autoswap_ max" sitevar.

 - Admins can override.

* Any user-requested idleswap duration is checked against the "idle/threshold"
  sitevar.

 - Admins can override.

* Disabling swapping (-S option) is prohibited, except by admins.

* Removed all special-case checks for requests coming via the XMLRPC server.