Move messages.c to libbb. Make each string in messages.c be its own .o file.

Move messages.c to libbb.  Make each string in messages.c be its own .o file.
This way, we can new get rid of all that tedious #define rubbish we used to
need to enable specific messages.  This way is enormously simpler, and as a
bonus also ends up saving us 96 bytes.
 -Erik