add ap_full_read() and ap_full_write(). they guarantee to read/write the

    full buffer unless an error occurs.
use the new functions in SDBM and DAV. [and Subversion]
Win32 and OS/2 can directly include/use file_io/unix/fullrw.c since it is
    written in terms of other APR functions.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@85850 13f79535-47bb-0310-9956-ffa450edef68