certificateMatchesProperties segfault

certificateMatchesProperties segfault

It is reported at least one case where squid crashed with segfault because the
signing certificate was NULL.

This patch:
  - Add assertion checks inside  buildSslCertGenerationParams and
    Ssl::certificateMatchesProperties functions (a)to avoid segfaults
  - In the case the signing certificate is not given in http_port configuration
    or the given certificate filename was not valid, squid does not start.
  - Creates the http_port_list::configureSslServerContext method and move
    here the cache_cf.cc code which was responsible to initialize ssl contexts
    and sslBump feature.