Merge r1212883:

Fix names of unary operators like '-s' being compared case insensitively.
Improve error message if op or function is not found. Document '-s' operator.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1212884 13f79535-47bb-0310-9956-ffa450edef68