Document os.h. I am only documenting the Unix copy, because every os.h

file has the same functions, and I don't want to duplicate docs.  When we
generate docs, we'll just grab them from this directory.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@86021 13f79535-47bb-0310-9956-ffa450edef68