[udesk/dante] Fix a misleading comment.

Fix the danteAllocSlice() comment, no error is
produced on allocation failure.
[udesk/dante] Ensure NULL slice field and proper static cache cleanup.

Make some extra effort to clear slice fields on context
initialization for fast statically reserved objects,
also ensure to clear them up on context destruction.
[udesk/dante] Perform some preprocessor checks and ensure a boolean value.

Perform some preprocessor check to ensure a C89 compatible compiler, also
ensure that chars are 8 bits wide. Introduce stdbool compatible
boolean types for dante, using the actual stdbool.h header whenever
possible.
[udesk/dante] Introduce a type checking function.

Introduce a function to check an object's type, in order
to reuse the same code for any udeskIsType kind of
function.