log: allow "tarantool" as module name in box.cfg{log_modules = ...}

Currently it's possible to set the log level for a particular Lua module,
or to change the default log level, however there is no way to change it
only for Tarantool system messages. This patch introduces a fake module
name "tarantool" for this purpose.

Closes #8320

NO_CHANGELOG=Unreleased feature
NO_DOC=Will update tarantool/doc#3264