Ticket 48163 - Read schema from multiple locations
    
    Bug Description:  We provide system schema that should not be altered or changed.
    This schema tends to be version specific to ds versions also. As a result, it
    would be good to move system schema to /usr, and keep custom or changing schema
    in /etc.
    
    Fix Description:  This adds a new configure option, systemschemadir, which
    provides the location of system schema to the schema.c file. Additionally,
    get priority filelist is altered to be able to prioritise over a number of
    directories to form the filelist.
    
    This leaves the only schema element in /etc as /etc/dirsrv/schema/99user.ldif
    
    https://fedorahosted.org/389/ticket/48163
    
    Author: wibrown
    
    Review by: mreynolds (Thanks!)