Refactor (without semantic changes) crypto/x509/{v3_purp.c,x509_vfy.c}

Refactor (without semantic changes) crypto/x509/{v3_purp.c,x509_vfy.c}

This prepares some corrections and improves readability (coding style).
Among others, it adds the static function check_sig_alg_match() and
the internal functions x509_likely_issued() and x509_signing_allowed().

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10587)