Change default RedHat confdir

RedHat sticks all mods/confs/vhosts in `$confdir/conf.d` but Debian
sticks them in separate directories. Previosly the Apache module
simulated Debian style conf dirs on RedHat, but this leads to confusing
purging and updating with respect to configs that are added to `conf.d`
by packages.

Changing the default to `conf.d` means that configs placed by packages
are updated rather than ignored or purged. Also if purging is turned off
this will not cause undesired config declarations.
Merge pull request #42 from hunner/change_redhat_default

Change default RedHat confdir