Distinguish between self-issued certificates and self-signed certificates.

Distinguish between self-issued certificates and self-signed certificates.
The certificate verification code has special cases for self-signed
certificates and without this change, self-issued certificates (which it
seems are common place with openvpn/easyrsa) were also being included in
this category.

Based on BoringSSL.

Thanks to Dale Ghent <daleg at elemental dot org> for assisting in
identifying the issue and testing this fix.

ok inoguchi@