filememallocator: Proper use of HAVE_DECL_* and debug category improvement

Apparently HAVE_DECL_* macros are always defined, either to 0 or 1 so the
conditional preprocessor expression needs to evaluate its value rather than
existance of the definition.

Improved file memory allocator debug category name (removed spaces and made
it more conventional).

Fixed a typo in the documentation example code.