address pid wraparound in hostuid_to_ns

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
set_value: start a trivial files blacklist

This is a list of files which clients should not be allowed to write
to.  For instance, tasks and cgroup.procs would allow bypassing
the extra restrictions imposed on MovePid, namely privilege of the
client over the victim.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>