Consolidate convenience macros/functions into one header file

MySQL Proxy uses a couple of convenience macros and functions that aren't available with a single #include directive.
Examples of these are the C() and S() macros, extended logging macros, glib-ext.h functions and certain lua extensions (although the lua ones must be separate to avoid tying everything together).
In client code, it should be as simple as including a single header file, to avoid having multiple copies of these macros around - right now there's a fair amount of "local" defines to achieve the same thing.