MEDIUM: boringssl: support native multi-cert selection without bundling

MEDIUM: boringssl: support native multi-cert selection without bundling

This patch used boringssl's callback to analyse CLientHello before any
handshake to extract key signature capabilities.
Certificat with better signature (ECDSA before RSA) is choosed
transparenty, if client can support it. RSA and ECDSA certificates can
be declare in a row (without order). This makes it possible to set
different ssl and filter parameter with crt-list.