Replace use of apr_file_write() with apr_file_write_full() to prevent

incomplete writes.

Add comments in some places where error handling/logging is missing.

PR: 53131.
Submitted by: Nicolas Viennot <apache viennot biz>, Stefan Fritsch


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1331110 13f79535-47bb-0310-9956-ffa450edef68