The cachers CLI thread is responsible for garbage collecting dicarded

The cachers CLI thread is responsible for garbage collecting dicarded
VCL programs, but in practice it didn't happen because we fired the
GC function VCL_Idle() after 5 seconds of silence on the CLI port and
the manager pings every three (by default).

Instead change the name of VCL_Idle() to VCL_Poll() and call it
before every CLI command, so that any query command will get an
up to date status.

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@2908 d4fa192b-c00b-0410-8231-f00ffab90ce4