Process signature algorithms before deciding on certificate.

The supported signature algorithms extension needs to be processed before
the certificate to use is decided and before a cipher is selected (as the
set of shared signature algorithms supported may impact the choice).
Reviewed-by: Matt Caswell <matt@openssl.org>

(cherry picked from commit 56e8dc542bd693b2dccea8828b3d8e5fc6932d0c)

Conflicts:
	ssl/ssl.h
	ssl/ssl_err.c