msg, client API: never send the status line as log message

The status line is a bit special; for example it uses special control
codes by design and is not terminated with a newline character in order
to update it on the terminal without scrolling. It's not helpful for
client API users either, and would require special-casing them
(emulating aspects of a terminal?). Also, the status line code is
explicitly disabled on osd.c unless the --terminal option is enabled,
so there was no good way to even enable the status line for the API.

Just pretend that the status line does not exist as far as the client
API is concerned. It won't be sent as MPV_EVENT_LOG_MESSAGE.