Introduce new API for flags of the SSL bufferevent
Introduce more generic API (like for evbuffer):
- bufferevent_ssl_set_flags()
- bufferevent_ssl_clear_flags()
- bufferevent_ssl_get_flags()
And deprecate existing:
- bufferevent_openssl_get_allow_dirty_shutdown()
- bufferevent_openssl_set_allow_dirty_shutdown()
- bufferevent_mbedtls_get_allow_dirty_shutdown()
- bufferevent_mbedtls_set_allow_dirty_shutdown()
Merge ssl implementations (openssl and mbedtls)
This patch splits common part out to avoid copy-paste from the
- bufferevent_openssl.c
- bufferevent_mbedtls.c
It uses VFS/bufferevent-like approach, i.e. structure of callbacks.
