Added support for includedir_path (disabled by default).

[jadestorm]

This PR adds support for includedir, which is in a lot of the krb5 default configs these days. Now, you might be asking.. why? Well, we have a semi bizarre section in our krb5.conf called appdefaults -- where settings for pam are placed. It looks something like this:

[appdefaults]
 pam = {
   debug = false
   ticket_lifetime = 50400
   renew_lifetime = 50400
   forwardable = true
   krb4_convert = false
   krb4_convert_524 = false
   krb4_use_as_req = false
   minimum_uid = 1
   afs_cells = first.example.org=afs/first.example.org@EXAMPLE.ORG second.example.org=afs/second.example.org@EXAMPLE.ORG third.example.org=afs/third.example.org@EXAMPLE.ORG
 }

Trying to account for anything that could possibly show up in that second seemed like an exercise in irritation, so I opted to go the route of "let folk enable includedir" and dump whatever they want in place.

I turned it off by default because while I checked Ubuntu, RHEL6, and RHEL 7, I have no idea how wide the support for includedir is and, really, most folk will want the module managing the -entire- config anyway. =)