Fix for data_dir relative path location (also corrects relative paths for themes...

Fix for data_dir relative path location (also corrects relative paths for themes, attach_dir, signature_page, and org_logo). Similar fix is still required in administration plugin in order for relative paths to be completely resolved.

This change to conf.pl does not create any incompatibility with previous versions, so I did not change the level - let me know if we should.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@3690 7612ce4b-ef26-0410-bec9-ea0150e637f0