Remove _BSD_SOURCE and _SVID_SOURCE

[Roland McGrath]

> Ping for the minimal patch 
> <https://sourceware.org/ml/libc-alpha/2013-12/msg00652.html>, now 2.19 is 
> out with _DEFAULT_SOURCE and the __FAVOR_BSD removal.  (Obviously a new 
> NEWS entry would go under the 2.20 section in NEWS rather than applying 
> the NEWS changes from that patch.)

I guess that is OK now.  The sys/ipc.h change should mention _GNU_SOURCE.
It's odd to mention _DEFAULT_SOURCE, since it's the default.  If we
encourage anything in particular, it should be _GNU_SOURCE.

I think before we simplify the __USE_* macro set, we should first use it as
a pointer toward obsolete interfaces we can deprecate or remove.  sigvec
and some of the other pre-POSIX signal functions come to mind.  Also
sys_errlist et al, since everything should be using strerror/strsigal by
now.  And audit of the exported symbols would be good.


Thanks,
Roland