src: dynamic input_descriptor allocation

src: dynamic input_descriptor allocation

This patch introduces the input descriptor list, that stores the
existing input descriptor objects. These objects are now dynamically
allocated and release from scanner_destroy() path.

Follow up patches that decouple the parsing and the evaluation phases
require this for error reporting as described by b14572f72aac ("erec:
Fix input descriptors for included files"), this patch partially reverts
such partial.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>