squid: Add RAM-only Proxy functionality

squid: Add RAM-only Proxy functionality

As suggested by Oliver "giller" Fieker <oli@new-lan.de>
in bug 10592 I added the functionality to use the squid as ram-only cache.

Further it defines the maximum_object_size_in_memory
as 2% of the in the webif defined "Memory cache size".
The maximum_object_size_in_memory should have a useful
size of the defined memory cache and I don't want to
create another variable which muste be fulled in by the user.

Signed-off-by: Daniel Weismüller <daniel.weismueller@ipfire.org>
Suggested-by: Oliver "giller" Fieker <oli@new-lan.de>
Suggested-by: Kim Wölfel <xaver4all@gmx.de>
Acked-by: Michael Tremer <michael.tremer@ipfire.org>
Cc: Stefan Schantl <stefan.schantl@ipfire.org>
Signed-off-by: Daniel Weismüller <daniel.weismueller@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>