Fix netdb.h addrinfo namespace (bug 18529)

[Roland McGrath]

> I've made the existing comment refer to POSIX.1:2001 instead of POSIX.1g 
> (a reference to POSIX.1:2001 seems the most useful in terms of the 
> standards supported by glibc).  I don't think the fact that there was once 
> a bug is particularly useful information to go in comments in the header.

The "bug" here was an explicit change in the API surface that affects the
build choices of code that is trying to be portable.  So I think it is
appropriate to make clear how the API has evolved over time.

> I think of these changes (that do not affect _DEFAULT_SOURCE or 
> _GNU_SOURCE and do not obsolete or remove any API) as much like any other 
> ordinary bug fix

I don't know why _GNU_SOURCE and _DEFAULT_SOURCE are special in your mind
for this subject.  The API for users of a different feature-test macro
state has changed, so the API has changed.