refactor: core code in lib/core and private-libwebsockets.h to core/private.h

This commit is coverity-clean as tested

cmake .. -DLWS_WITH_MINIMAL_EXAMPLES=1 -DLWS_WITHOUT_EXTENSIONS=1 -DLWS_WITH_ACME=1 -DLWS_WITH_LWSWS=1 -DLWS_WITH_LIBUV=1 -DLWS_WITH_HTTP2=1 -DLWS_WITHOUT_CLIENT=0 -DLWS_WITHOUT_SERVER=0 -DLWS_UNIX_SOCK=1 -DLWS_WITH_TLS=0 -DLWS_WITH_MBEDTLS=0 -DLWS_WITH_CGI=1 -DCMAKE_BUILD_TYPE=DEBUG -DLWS_WITH_GENERIC_SESSIONS=1 -DLWS_WITH_RANGES=1 -DLWS_ROLE_WS=1 -DLWS_MAX_SMP=16 -DLWS_ROLE_H1=1 -DLWS_WITH_WOLFSSL=0 -DLWS_WITH_LIBEV=0 -DLWS_WITH_LIBEVENT=1