add apr_get_home_directory(), teach mod_userdir to use that

instead of calling getpwnam[_r] directly, back out mod_userdir's
config check for getpwnam_r


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@86932 13f79535-47bb-0310-9956-ffa450edef68