mxml: add optional/mandatory flag to nft_mxml_reg_parse

There are some cases where a reg is not mandatory, for example:
 * dreg in lookup
 * dreg/sreg in meta (last version)

So, lets change the function nft_mxml_reg_parse() to add
an optional/mandatory flag.

dreg in lookup is optional as stated at:
 net/netfilter/nft_lookup.c:nft_lookup_init()

Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
lookup: xml: conditional output of dreg

The dreg attribute is optional as stated at:
linux/net/netfilter/nft_lookup.c

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