GnuTLS: Always send client cert

GnuTLS: Always send client cert

TLS servers may request a certificate from the client. This request
includes a list of 0 or more acceptable issuer DNs. The client may use
this list to determine which certificate to send. GnuTLS's default
behavior is to not send a client certificate if there is no
match. However, OpenSSL's default behavior is to send the configured
certificate. The `GNUTLS_FORCE_CLIENT_CERT` flag mimics OpenSSL
behavior.

Authored-by: jethrogb on github
Fixes #1411
Closes #4958