Abstract read() and write() to the ap library to make it easier

to add in different types of read/write libraries in a central
location. This mainly keeps buff.c less messy with #ifdefs.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@80081 13f79535-47bb-0310-9956-ffa450edef68