netfilter: nat: merge ipv4/ipv6 masquerade code into main nat module

Instead of using extra modules for these, turn the config options into
an implicit dependency that adds masq feature to the protocol specific nf_nat module.

before:
   text    data     bss     dec     hex filename
   2001     860       4    2865     b31 net/ipv4/netfilter/nf_nat_masquerade_ipv4.ko
   5579     780       2    6361    18d9 net/ipv4/netfilter/nf_nat_ipv4.ko
   2860     836       8    3704     e78 net/ipv6/netfilter/nf_nat_masquerade_ipv6.ko
   6648     780       2    7430    1d06 net/ipv6/netfilter/nf_nat_ipv6.ko

after:
   text    data     bss     dec     hex filename
   7245     872       8    8125    1fbd net/ipv4/netfilter/nf_nat_ipv4.ko
   9165     848      12   10025    2729 net/ipv6/netfilter/nf_nat_ipv6.ko

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>