Usually posix functions have individual doc pages, and each header has its own
list of required macro definitions. Use a simpler key of "in-latest-posix" to
signal that the URL convention can be followed.
Add support for a "removed-in-posix-2008" key which will link to the 2004 docs
for functions like bcmp, bcopy, bzero, index, and rindex from strings.h.
I don't want to add all of these links for pthreads.h, so automating this will
make documenting these go much faster.
