Add support for conditional logging depending on an expression.

The syntax is a bit unwieldy, the quotes have to start before
the 'expr=':

    CustomLog "logs/cond_log" combined "expr=req('User-Agent') == 'x'"


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1033157 13f79535-47bb-0310-9956-ffa450edef68