do not abstract OpenSSL, to let the users customize the configuration by directly calling its functions
update examples/websocket.c following the changes in prev. commit
Merge branch 'kazuho/refactor/no-abstraction-for-openssl' into kazuho/expr/ssl-configuration
add ssl-related configuration directives to `listen`
suppress deprecated warnings on osx
merge branch 'kazuho/expr/ssl-configuration' (#18)