fix(packet): drop capabilities permanently

[Komzpa]

Summary

• drop all mtr-packet process capabilities after privileged socket setup
• stop keeping CAP_NET_RAW / CAP_NET_ADMIN in the permitted set for later re-enabling
• call setsockopt() directly for SO_MARK / SO_BINDTODEVICE instead of temporarily raising effective capabilities around those calls
• add a regression test that rejects future packet-helper code paths that select or re-enable individual capabilities

Why

This follows the security model discussed in #311, especially the concern in #311 (comment): if packet-handling code can later re-enable a capability from the permitted set, then the privilege was not really dropped for exploit-mitigation purposes.

mtr-packet already opens its raw sockets before dropping elevated permissions. After that point, later command handling should not be able to regain capabilities. Options that need privileges later will now fail normally if the process no longer has them, rather than silently re-enabling them.

This is intentionally not a cherry-pick of #311: that PR moved toward permitted-but-not-effective capabilities. This PR takes the stricter direction requested in the review discussion.

Stack note

#611 has been rebased on top of this PR so low local-port validation and error reporting compose with permanent capability dropping. The combined behavior does not re-enable CAP_NET_BIND_SERVICE: low privileged ports either succeed because the OS permits the bind without restored capabilities, or fail with a focused permission error.

Validation

• ./bootstrap.sh && ./configure --without-gtk --without-jansson && make -j$(nproc)
• git diff --check
• make check TESTS='test/capability-drop.py'
• sudo python3 ./test/cmdparse.py
• ./mtr -r -c 1 127.0.0.1
• composite newest-to-oldest stack including fix(packet): drop capabilities permanently #647 and the rebased fix(packet): allow privileged local ports #611/test(dist): cover tarball version metadata #638/fix(curses): show decimal latency scale #637/test(curses): cover large count formatting #636/feat(curses): add compact startup and scale controls #580/feat(curses): show host and protocol in title #576/csv: omit trailing empty header field #575 tail: clean merge
• ./bootstrap.sh && ./configure --without-gtk --without-jansson && make -j$(nproc) on that composite stack
• git diff --check on that composite stack
• make check TESTS='test/capability-drop.py test/dist-version.sh format-count-test' on that composite stack
• sudo python3 ./test/cmdparse.py on that composite stack
• sudo python3 ./test/probe.py TestProbeUDP on that composite stack

Supersedes #311.