Determine whether or not a particular header was covered by a specific
signature on a message.
||dkim_sig_hdrsigned() can be called at any time after
headers have been processed for a message being signed or
verified, essentially after a call to
||The name of the header to be checked.
||The header named by hdr was included in the
signature referenced by sig.
||sig did not cover hdr.
- The check performed by this function is fairly simple: it will
see if hdr appears in the "h=" tag for the signature
identified by sig. It does not guarantee that a specific
instance of that header was signed, which is relevant for headers
that appear more than once (e.g. Received:).