Standardize on bool/true/false (lowercase) in the C source

We still build some components (e.g. xpdev) with pre-C99 Borland C, so we still use BOOL/TRUE/FALSE in these source/header files for that reason. Since we know that bool/true/false is the actual/future standard (C23), it'll be safe to use those keywords and macro-wrappers for any compiler (e.g. Borland C) that doesn't have native support.

This will make editing the C source files simpler since we'll know to always use the lowercase variant, regardless of which module/lib, without any surprises (e.g. when the CI pipeline builds code with Borland C).

Need to make sure the Borland C wrapper for bool will be the same size as the C99 stdbool.h/_Bool type.