Overhaul the regexp purge/ban code, with a detour around the CLI code.


CLI code:

	In CLI help, don't list commands with no syntax description.

	Add a CLI_HIDDEN macro to construct such entries.  They are
	useful as backwards compatibility entries which we do not want
	to show in the help.

CLI interface to BAN (purge) code:

	Get the CLI names right for purging so they are purge.FOO instead
	of FOO.purge.

	This means that you should use "purge.url" and "purge.hash"
	instead of "url.purge" and "hash.purge".

	Add compat entries for the old, and keep them through the 2.x
	release series.
	

	Add purge.list command to list purges currently in effect.
	NB: This is not 100% locking safe, so don't abuse it.


BAN (purge) code:

	Add reference counting and GC to bans.

	Since we now have full reference counting, drop the sequence
	number based soft references and go to "hard" pointer
	references from the object to the purges.

	Give the "ban" structure the miniobj treatment while we are
	at it.

	The cost of this is a lock operation to update refcounts
	once all applicable bans have been checked on an object.

	There is no locking cost if there is no bans to check.

	Add explicit call to new BAN_DestroyObj() when objects are
	destroyed to release the refcount.

	When we release an object refcount in BAN_DestroyObj(),
	check if we can destroy the last purge in the list also.

	We only destroy one ban per BAN_DestroyObj() call, to avoid
	getting stuck too long time, (tacitly assuming that we will
	destroy more objects than bans.)



git-svn-id: http://www.varnish-cache.org/svn/trunk@2645 d4fa192b-c00b-0410-8231-f00ffab90ce4