Rename 'purges' to 'bans' so that we can implement _real_ purges
also.

They really have been called 'bans' internally all the way through,
but for reasons now forgotten, we thought it would be too hard to
explain to people that purges were called bans in Varnish, so
the user-fronting side were named purges.

As it transpired, people were surprised that purges in varnish
did not free the storage right away, and various other kinds of
confusion also materialized.

The sky convinced me to implement real purges for efficiency
reasons when Vary is used, and of course now the name "purge"
was taken.

I'm sure people will curse about this change in the 2.x to 3.x
migration, but sooner or later it had to happen, and now it did.



git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@5672 d4fa192b-c00b-0410-8231-f00ffab90ce4