BUG/MAJOR: map: fix segfault during 'show map/acl' on cli.

BUG/MAJOR: map: fix segfault during 'show map/acl' on cli.

The reference of the current map/acl element to dump could
be destroyed if map is updated from an 'http-request del-map'
configuration rule or throught a 'del map/acl' on CLI.

We use a 'back_refs' chaining element to fix this. As it
is done to dump sessions.

This patch needs also fix:
'BUG/MAJOR: cli: fix custom io_release was crushed by NULL.'

To clean the back_ref and avoid a crash on a further
del/clear map operation.

Those fixes should be backported on mainline branches 1.7 and 1.6.

This patch wont directly apply on 1.6.