Process signature algorithms before deciding on certificate.

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