canbus: fix frame comparison semantics

[Opriego]

Summary

Fix canbus_framecmp() so CAN frames are compared consistently and as binary data.

The previous implementation had three issues:

• It returned 1 when CAN IDs differed, but also returned 1 when payloads were equal, resulting in inconsistent comparison semantics.
• It used strcmp() for CAN payloads, even though CAN data is binary and may contain embedded NUL (0x00) bytes.
• It did not compare can_dlc, so frames with different payload lengths could be treated incorrectly.

Changes

• Compare CAN IDs explicitly.

• Compare DLC values explicitly.

• Replace strcmp() with memcmp() over exactly can_dlc bytes.

• Keep comparison semantics consistent:

  • 0 = frames are equal
  • non-zero = frames are different

• Add regression tests covering:

  • identical frames
  • different CAN IDs
  • different DLC values
  • different payloads
  • binary payload differences after an embedded NUL byte

Testing

The regression suite was first run against the original implementation:

Running suite(s): canbus
60%: Checks: 5, Failures: 2, Errors: 0

The failures demonstrated the inconsistent comparison behavior for identical frames and differing payloads.

After applying the fix:

Running suite(s): canbus
100%: Checks: 5, Failures: 0, Errors: 0

The new check_canbus target was also validated through Automake generation, confirming that both tests/check_canbus.c and src/canbus.c are included in the test executable.