bfcli: fix parsing log flags

[pzmarzly]

Stack from ghstack:

• daemon: validate enum values coming from user #368
• bfcli: fix parsing bf_hook enum #367
• -> bfcli: fix parsing log flags #366

ReviewStack: https://reviewstack.dev/facebook/bpfilter/pull/366

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ctest --test-dir build -R e2e.daemon.netns_to_host was flaky, which was annoying me, as it was failing on unrelated changes.

With some Claude-assisted debugging, I found out it's because uint8_t log sometimes contains garbage data, that we later OR into in log |= BF_FLAG(header); line. When that happens, the printed output doesn't match what awk is expecting.