MINOR: ssl: allow to change the signature algorithm for client authentication

MINOR: ssl: allow to change the signature algorithm for client authentication

This commit introduces the keyword "client-sigalgs" for the bind line,
which does the same as "sigalgs" but for the client authentication.

"ssl-default-bind-client-sigalgs" allows to set the default parameter
for all the bind lines.

This patch should fix issue #2081.