libxtables: add xtables_ip[6]mask_to_cidr

libxtables: add xtables_ip[6]mask_to_cidr

This patch adds generic functions to return the mask in CIDR
notation whenever is possible.

This patch also simplifies xtables_ip[6]mask_to_numeric, that
now use these new two functions.

This patch also bumps libxtables_vcurrent and libxtables_vage
since we added a couple new interfaces (thanks to Jan Engelhardt
for his little reminder on this).

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