libc fortify: make unistd.h use diagnose_if

Also:
- remove a NULL check. The bug that's been working around has been
  fixed since clang r298431, and our compiler is built from r300080, and
- unwrap some function prototypes, per b/36984245.

If you're interested, here's what a new error versus an old one looks
like: https://drive.google.com/file/d/0B-8OBPboNJ_4MVdlYXdJbUFBYUk/view

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

Change-Id: Ibafe61198988b4c8bee8391bcdf01bcbc04233de