set: add XML parsing

set: add XML parsing

Sets are now parsed, following this previous snprintf pattern:

<set>
	<set_name>string</set_name>
	<set_table>table</set_table>
	<set_xml_version>int</set_xml_version>
	<set_flags>uint32_t</set_flags>
	<key_type>uint32_t</key_type>
	<key_len>size_t</key_len>
	<data_type>uint32_t</data_type>
	<data_len>size_t</data_len>
	<set_elem>
		<set_elem_flags>uint32_t</set_elem_flags>
		<set_elem_key>
			<data_reg type="value">
				<len></len>
				<dataN></dataN>
			</data_reg>
		</set_elem_key>
		<set_elem_data>
			<data_reg type="xx">
				[...]
			</data_reg>
		</set_elem_data>
	</set_elem>
</set>

Signed-off-by: Arturo Borrero González <arturo.borrero.glez@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>