get_mempolicy.2, set_mempolicy: Policy is per thread, not per process

get_mempolicy.2, set_mempolicy: Policy is per thread, not per process

set/get_mempolicy manpages say that the memory allocation
policy is per process while reading the code and testing shows
that it's actually per thread.  Here's a quick fix, which may
need to be improved to better explain that we're allocating
in the context of a thread within a process address space.

Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>