libc fortify: make string.h use diagnose_if

This also has a handful of style fixups, to make this file more
consistent. And removes __bionic_zero_size_is_okay_t, since there's a
better workaround available.

Bug: 12231437
Test: m checkbuild on bionic internal master; CtsBionicTestCases show
no new failures.

Change-Id: I75a020630dbab0ce828563502900cba14ae992d1