PI_FETCH_TASKINFO to proc_getprocinfo and through
_hurd_priority_which_map.
* sysdeps/mach/hurd/setpriority.c (setpriority): Pass new arg to
_hurd_priority_which_map.
* hurd/hurdprio.c (_hurd_priority_which_map): New arg PI_FLAGS,
passed through to proc_getprocinfo RPC.
* hurd/hurd/resource.h: Add PI_FLAGS arg in decl.