feat: add walk-mode selection and cross-mode comparison to opendmarc-check

[thegushi]

Summary

• feat: add -m (auto/psl/rfc7489/rfc9989) and -p (Public Suffix List file) to opendmarc-check, exposing the DMARCbis walk_mode knob that opendmarc.c already reads from DMARCbisWalkMode but opendmarc-check never let you pick
• feat: add -a to query a domain under all four walk modes in one pass and print a comparison line per mode (status, organizational domain, p=, sp=, whether the RFC 7489 fallback walk was used)
• refactor: extract policy_to_str()/alignment_to_str() helpers from the existing inline switch statements so the classic single-domain report and the new comparison output share one source of truth

This goes through the actual opendmarc_policy_query_dmarc() path the milter uses, not a re-implementation, so it can be used to spot real divergence between walk modes for a given label instead of relying on the Perl survey scripts in contrib/.

Test plan

• make check in libopendmarc passes (11/11 tests)
• Clean build, no compiler warnings
• Manually queried live domains (google.com, mail.google.com) with -m, -a, and -a -p ; confirmed psl/auto/rfc7489/rfc9989 agree when a PSL is loaded and diverge sensibly when it isn't
• Confirmed -m and -a are rejected together