capsicum-test: make the code WARNS=6 clean

[ngie]

Summary

Make the code WARNS=6 clean by doing the following:

• Renaming variables (-Wshadow).
• Use memcpy instead of assigning the result of CMSG_DATA to a variable (-Wcast-align).
• Wrap some macros with do-while(0) to avoid variable collisions (-Wshadow).
• Push one of the TRACE statements up by 1 layer to avoid a collision between 2 calls of TRACE via the same overarching macro.

This was needed in order for the code to build properly with D61066,
since the code was not compiling with WARNS set to 3 at the testcase
level.

MFC after: 1 week

Test Plan

• Compiles and runs successfully with amd64:

% sudo kyua test -k /usr/tests/sys/capsicum/Kyuafile capsicum-test_unprivileged capsicum-test_privileged
...
Results file id is usr_tests_sys_capsicum.20261031-195148-317989
Results saved to /root/.kyua/store/results.usr_tests_sys_capsicum.20261031-195148-317989.db

209/216 passed (0 broken, 0 failed, 7 skipped)

All of the skips are because some of the tests are run with an unprivileged user, but require root, vice versa, or because I don't have SCTP support in my VM.

• make tinderbox in-progress.

[ngie]

Move WARNS to this review/commit from D61066.

[markj]

In places where you're fixing variable shadowing, it would be better to give new names to both the parent and child's variables, so that the code is clearer and mistakes are more likely to be caught.

tests/sys/capsicum/socket.cc

295

This should test sock2.

317

Zeroing the wrong addr here.