libc fortify: make stdio.h use diagnose_if

This also throws in some reformatting to match the GCC FORTIFY style in
this file (e.g. 4 space standard indent).

Per b/36984245, some function protos are unwrapped beyond 100 chars, as
well.

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

Change-Id: I6dce60ec14d5cd703d9e724699634a197ca11023