sched_policy: split add_tid_to_cgroup

add_tid_to_cgroup was also being used to add threads to
cpusets.  Split this into a generic write_tid_to_fd, and
wrap it for cpusets and cgroups.  This lets errors align
with what the actual failure is.

Change-Id: I2c5c97117428a33b6a328f73741d3516d4141f78