CLEANUP: resolvers: use read_n32() instead of open-coded big-endian read

CLEANUP: resolvers: use read_n32() instead of open-coded big-endian read

In resolv_validate_dns_response(), the second DNS record parsing path
manually constructs a 32-bit big-endian TTL value from four individual
bytes using the expression:

  reader[0] * 16777216 + reader[1] * 65536 + reader[2] * 256 + reader[3]

We have read_n32() to do this, and it's more robust against unexpected
signedness surprises (which should not happen right here since reader is
unsigned char and we use -fwrapv so the result is defined). Also, let's
make the ttl an uint instead of an int. The TTL is only retrieved and not
used for now, so better clean it now.