fix: validate ICMP ID per socket, fix dual-stack ID mismatch

[gsnw-sebast]

Fix three related bugs around ICMP echo identifier handling:

1. recv_ping() did not filter incoming packets by expected ID.
   In SOCK_DGRAM mode the Linux kernel overwrites the ICMP identifier with
   the port number assigned by bind(), so a post-call check against a
   self-chosen process ID would silently discard all legitimate replies or
   accept replies from other processes.

2. pinger.rs tracked a single my_id for both sockets via
   dgram_id4.or(dgram_id6). In dual-stack operation each SOCK_DGRAM socket
   receives its own independent kernel-assigned port; collapsing them into
   one value caused all IPv6 echo replies to be rejected because their
   assigned ID differed from the IPv4 one stored in my_id.

3. build_icmp_packet() was called with the now-removed shared my_id
   instead of the per-address-family ID, so sent packets already carried
   the wrong identifier before any reply was received.