Switch per-device attribution from SO_BINDTODEVICE to IP_PKTINFO

SO_BINDTODEVICE pins both ingress *and* egress to the bound
interface — the kernel uses the listening socket's device
binding when choosing the output interface for the SYN-ACK,
which is sent before accept() returns and therefore can't be
fixed up in userspace. That's fatal for maglev / DSR
deployments where the SYN arrives through a GRE tunnel but the
return path has to leave via the default route; the SYN-ACK
goes out the GRE and is dropped by the uplink, so every new
connection times out.

Rework the listen plumbing so the module never touches
SO_BINDTODEVICE. init_module now enables IP_PKTINFO and
IPV6_RECVPKTINFO on every HTTP listening socket and resolves
each configured `device=` name to an ifindex. At request time
resolve_source calls getsockopt(IP_PKTOPTIONS) on the accepted
fd to read the per-connection in(6)_pktinfo cmsg the kernel
stashed during the handshake, then matches (ifindex, family)
against the bindings table. The listening sockets remain plain
wildcards, so the return path follows the normal routing table
and DSR works.

The wrapper also no longer clones or rebinds sockets: it still
dedups per (cscf, sockaddr) so multiple device-tagged listens
in a single server block coexist, and dedups bindings on
(device, family) so the same device can carry different tags
for v4 and v6 (e.g. tag2-v4 / tag2-v6) but not pointlessly
duplicate when a listen include is shared across server blocks.

Drive-by fixes to unblock `make pkg-deb` after a prior
`make build-asan`:
- debian/rules overrides dh_clean to exclude build/, since
  nginx-asan's install creates nobody:0700 temp dirs dh_clean
  can't traverse.
- Makefile's build-asan removes those unused runtime temp dirs
  so the tree is clean afterwards.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

debian/rules

@@ -36,3 +36,11 @@ override_dh_auto_clean:
 	# Makefile's `clean` target, which wipes build/ wholesale — and
 	# that includes build/nginx-asan/ from a prior `make build-asan`.
 	# Users who want a fresh reset run `make clean` at the top level.
+
+override_dh_clean:
+	# `dh_clean` recurses from the package root to remove junk files
+	# (editor backups, autom4te caches, etc.). `make build-asan`
+	# produces build/nginx-asan/{fastcgi,proxy,scgi,uwsgi}_temp owned
+	# by "nobody" with mode 0700, which the current user can't
+	# traverse — so we exclude anything under build/ from dh_clean.
+	dh_clean -X build/