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.
Use of `strcpy` in these particular cases was safe, but it's hard to
tell and also useless. `strcpy` would effectively need to do another
`strlen` which we already did.
Also removed sprintf, which was also safe in this case but it's easier to
be "obviously safe", especially for static analysers.
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.
The android warnings are disabled now because they suggest using
linux-only extensions of libc. Useful for android indeed, but we're
targeting non-android and non-linux systems as well.
