This ensures notably that libnss_sss looks for the mmap cache files in
the right place, /var/lib/sss/mc, rather than in the store.
* gnu/packages/patches/sssd-system-directories.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.
* gnu/packages/sssd.scm (sssd)[source]: Use it.
[arguments]: Pass "--localstatedir" and "--sysconfdir".
