compositor: Create the 'log' scope much earlier

Logging should start as early as possible so create the log scope as
early as possible, before subscribing to it.

Open the logfile before creating the 'logger' subscriber, making sure
we're logging to the file properly.

Also migrate `weston_log_set_handler()` to avoid potential calls to
`weston_log` before installing the log handler.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>