Change ssl_sigalgs_build() to perform sigalg list selection.

Change ssl_sigalgs_build() to perform sigalg list selection.

Rather that doing sigalg list selection at every call site, pass in the
appropriate TLS version and have ssl_sigalgs_build() perform the sigalg
list selection itself. This reduces code duplication, simplifies the
calling code and is the first step towards internalising the sigalg lists.

ok tb@