Quote log-sink arguments for {Custom,Error,Transfer}Log directive.

Except syslog and journal, which are keywords.  There's no harm
in quoting those, but no real point to it, either.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1674195 13f79535-47bb-0310-9956-ffa450edef68