newpass_p12(): factor for loop body into helpers

newpass_p12(): factor for loop body into helpers

Since newpass_bags() and sk_PKCS7_push() could be shared between two
otherwise entirely unrelated code paths, it was decided to dedup the
code in about the ugliest possible way. Untangle the spaghetti and
split the code paths into helper functions, so we can easily error
check and avoid a bunch of leaks.

ok jsing