issue #233: opendkim: Use "policy" for dkim result of ignored sig, instead of "fail"

[futatuki]

FIx issue #233: use "dkim=policy" for the result of DKIM signature which is marked as "ignore" by dkim_sig_ignore(), instead of "dikim=fail".

• opendkim/opendkim.c (dkim_ar_all_sigs)
  • as above
  • reduce dkim_sig_getflags() calls