[SSSD][PATCH] make globals in *_opts.h extern

[Pavel Březina]

This solves situation where you want to use those globals on other place 
than in *_common.c.

I also created https://fedorahosted.org/sssd/ticket/2890 so we can avoid 
order-dependency on header files such as sysdb_services.h which I had to 
fix for AD patch.