MFC r312934:
Make use of clang nullability attributes in C headers.

Replace uses of the GCC nonnull attribute with the clang nullability
qualifiers. These are starting to get use in clang's static analyzer.

Replacement should be transparent for developers using clang. GCC ports
from older FreeBSD versions may need updating if the compiler was built
before r312860 (Jan-27-2017).

Hinted by: Apple's Libc-1158.20.4, Bionic libc

Relnotes: yes