Improve <sys/cdefs.h>.

Fix and use __RENAME (and lose ___RENAME --- two underscores should be
enough for anybody). This was the point of this change, because I want
to use __RENAME to support the two basename variants and the two
strerror_r variants.

Lose a bunch of macros that weren't being used.

Lose three dead files from the DNS code.

Change-Id: I3ef645c566b16a52217bc2e68c7d54b37c7c9522