src: xml: consolidate common XML code via nft_mxml_num_parse

src: xml: consolidate common XML code via nft_mxml_num_parse

This patch moves common XML parsing code to nft_mxml_num_parse().
To handle this, the nft_strtoi() helper fuction is included.

I've changed some MXML_DESCEND[_FIRST] flags to avoid match a nested node under
some circumstances, ie, matching two nodes with the same name that are descendant.

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