These help static analysis and ubsan. We should eventually have all
functions annotated like this with a cimple check to make sure every
pointer has an explicit nullability annotation. The `nullable`
annotation does nothing in GCC, but will be used by cimple to validate
that every parameter has defined nullability.
Currently: 1) libsodium and 2) nacl.
Note that the "nacl" variant is actually libsodium. We just want to make
sure the static analysers see the `VANILLA_NACL` code paths.
This flag isn't helpful for the most common case of aggregate
initialisation, namely `{0}`. We don't want to be writing `{{{0}}}` with
some brittle number of braces.
We used to have lots of these in the code, but now that all the endian
stuff is no longer dependent on host byte order, we can re-enable the
warning flag and catch any future violations.
