Allow certificates with Basic Constraints CA:false, pathlen:0

Allow certificates with Basic Constraints CA:false, pathlen:0

Do not mark such certificates with EXFLAG_INVALID although they
violate the RFC 5280, they are syntactically correct and
openssl itself can produce such certificates without any errors
with command such as:

openssl x509 -req -signkey private.pem -in csr.pem -out cert.pem \
  -extfile <(echo "basicConstraints=CA:FALSE,pathlen:0")

With the commit ba4356ae4002a04e28642da60c551877eea804f7 the
EXFLAG_INVALID causes openssl to not consider such certificate
even as leaf self-signed certificate which is breaking existing
installations.

Fixes: #11456
Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/11463)