build: finish fixing Linux TX_RING support after #1043

[fklassen]

Summary

Fixes #1044.

#1043 (thanks @plusky) fixed a typo in src/common/txring.h's include guard - __GLIBC_MINOR instead of __GLIBC_MINOR__ always evaluates false, so the #else branch (missing the TX_RING API entirely) was taken unconditionally on every glibc system - and switched to <linux/if_packet.h>, the header that actually defines struct tpacket_hdr/tpacket_req/TPACKET_HDRLEN.

That fix alone wasn't enough to actually enable TX_RING. This PR finishes the job with three more fixes surfaced by actually verifying it end-to-end (build + sudo make test, not just "does it compile in isolation"):

1. Both build systems' TX_RING probes had the same header collision

configure.ac and cmake/ConfigureChecks.cmake's HAVE_TX_RING probe test programs independently included <netpacket/packet.h> alongside <linux/if_packet.h>. The two cannot be included together before C23 - both define struct sockaddr_ll/packet_mreq, and the __UAPI_DEF_* de-duplication guards don't apply pre-C23 (verified directly - reproduces the identical redefinition errors on a real -std=gnu17 system). So the probe itself failed to compile, meaning HAVE_TX_RING was never defined in the first place on affected systems (the openSUSE/Fedora-style "older language level" case #1043 called out) - txring.h's now-correct code was never even reached. Dropped the redundant <netpacket/packet.h> from both probes; it supplies nothing they use.

2. Fixing the probes surfaced a third instance of the same collision

Since HAVE_TX_RING had literally never been true in any build before this chain of fixes, this code path had never been exercised: src/common/sendpacket.h and sendpacket.c each unconditionally include <netpacket/packet.h> for struct sockaddr_ll whenever HAVE_PF_PACKET is set (in sendpacket.c, also whenever HAVE_LIBURING is set) - both commonly true alongside HAVE_TX_RING on a modern Linux system - which then collided with txring.h's <linux/if_packet.h>. Made those three includes conditional on !HAVE_TX_RING, since txring.h already provides struct sockaddr_ll via <linux/if_packet.h> in that case.

Test plan

• Fresh clone, autotools: Linux TX_RING: yes correctly detected (was no before this PR, yes in the probe but build failures if only txring: fix __GLIBC_MINOR typo and include the header that defines the TX_RING API #1043's fix were applied), full build succeeds with zero errors, sudo make test passes
• Fresh clone, CMake: same - HAVE_TX_RING succeeds, full build succeeds with zero errors
• Runtime: tcpreplay --version reports Injection method: PF_PACKET / TX_RING; actually replayed packets through it (179/179 successful, 0 failed), throughput ~13x over plain PF_PACKET send()
• Confirmed via grep that no other file in the tree includes both <netpacket/packet.h> and <linux/if_packet.h> (or has an unguarded include of one that could still collide) after this fix

Also updates docs/CHANGELOG and docs/CREDIT to credit @plusky for #1043.

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