Allow to continue on UNABLE_TO_VERIFY_LEAF_SIGNATURE

Allow to continue on UNABLE_TO_VERIFY_LEAF_SIGNATURE

This unifies the behaviour of a single certificate with
an unknown CA certificate with a self-signed certificate.
The user callback can mask that error to retrieve additional
error information. So the user application can decide to
abort the connection instead to be forced by openssl.

This change in behaviour is backward compatible as user callbacks
who don't want to ignore UNABLE_TO_VERIFY_LEAF_SIGNATURE will
still abort the connection by default.

CLA: trivial
Fixes #11297

Reviewed-by: David von Oheimb <david.von.oheimb@siemens.com>
Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/11359)