build: improve const correctness and prototype declarations

build: improve const correctness and prototype declarations

String literals aren't assigned to non-const char pointers anymore, they
aren't returned via non-const char pointers, const strings aren't passed
arround via non-const pointers etc.

As a side effect, this eliminates all -Wdiscarded-qualifiers warnings.

Symbol visibility is minimized by declaring additional functions
static, i.e those which are only used inside a single translation unit.

Also, a few includes are added such that all non-static function
definitions 'see' their declarations - such that accidental differences
in signatures immedialy yield compile errors or warnings.

A few macros are also modified to emit forward declarations for
non-static functions in order to eliminate noise when compiling with
-Wmissing-prototypes.
However, it might make sense to move those non-static function
definitions from a header into a dedicated translation unit,
at some point.