Improve multiline highlighting via font-lock-syntactic-keywords

This improvement means that:
- there's no delay before multiline literals are recognized
- fontification can be controlled in detail via font-lock setup
- syntactic properties are always as up-to-date as possible

Basically, it's the same refactoring as in dbedb1e, but backported to
support Emacs23.  Emacs24 version is faster though, because font-lock
updates buffer line-by-line while `syntax-propertize' does that in
blocks of 500 characters.
Clean up lua-mode start-up function a bit
Fix incorrect error highlighting in 'for x = ...' statements
Print 'Prefix key set to ...' only when setting via interactive function
Don't unindent comment close-bracket if it doesn't match open-bracket
Bump TODO