All Source Files: Lots of header/macro/function cleanups

All Source Files: Lots of header/macro/function cleanups

- Consistently use 2 spaces for preprocessor directive indentation

- Ensure all header files can successfully compile stand-alone

- Rearrange inclusions in alphabetical order; now that each header
  can build by itself, it doesn't matter what order you use them in

- Replace compiler attributes macro header wholesale with the one from
  Atheme IRC Services

- Add lots of missing function attributes where applicable

  Memory allocation attributes in particular will be useful for
  static analysis; but you can't just add the malloc attribute to
  every function returning a newly-allocated pointer. The malloc
  attribute is used to declare functions that return newly-allocated
  memory, and if that return value is a pointer to a newly-allocated
  structure, and some of its members point to memory that existed
  before the function was called, then it is not entirely newly-
  allocated memory, and the optimizer might generate broken code
  with the faulty assumption given to it. So, a careful audit was
  required to identify functions that allocated memory without setting
  pointers in the resulting structures to point to things that existed
  before the allocating function was called.

- Remove unnecessary includes from source files

- Fix 2 broken parameter-less function prototypes

- Avoid using "#elif" preprocessor directive where it does not result
  in uglier output