Simplify ap_read_pid and make it more portable.

- Switch to using apr_file_read_full()
- Stop checking for \n (non-portable)
- Error if we read the entire buffer or the first digit isn't a number


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@95338 13f79535-47bb-0310-9956-ffa450edef68