mod_userdir: Constify and save a few bytes in the conf pool when parsing

"UserDir" directive

Submitted by: jailletc36
Reviewed by: jim, minfrin


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1748332 13f79535-47bb-0310-9956-ffa450edef68