MINOR: config: add global directives to set default SSL ciphers

The ability to globally override the default client and server cipher
suites has been requested multiple times since the introduction of SSL.
This commit adds two new keywords to the global section for this :
  - ssl-default-bind-ciphers
  - ssl-default-server-ciphers

It is still possible to preset them at build time by setting the macros
LISTEN_DEFAULT_CIPHERS and CONNECT_DEFAULT_CIPHERS.