netfilter: use nf_conntrack_helpers_register when possible

amanda_helper, nf_conntrack_helper_ras and nf_conntrack_helper_q931 are
all arrays, so we can use nf_conntrack_helpers_register to register
the ct helper, this will help us to eliminate some "goto errX"
statements.

Also introduce h323_helper_init/exit helper function to register the ct
helpers, this is prepared for the followup patch, which will add net
namespace support for ct helper.

Signed-off-by: Liping Zhang <zlpnobody@gmail.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>