fopen_proc
namespace: don't fail if a namespace isn't supported by kernel

CRIU reads /proc/pid/ns/[NS] and fails of a link is not exist.
setuid
Defining a mapping is a one-time operation per namespace
userns

time bash test/zdtm.sh  -i 0 -x cwd -x maps -x sched -x sock -x fanot ns/.*
xxx
devices
hack: pivot_root
sysctl: hack
hack: devpts
shmem: use chunk mode for dumping shared memory

Before this patch, criu splices all data in pipes and then saves these
data in a image file. Here is a problem, becase creating pipes with big
buffers fails too often, because a kernel tries to allocate a big linear
chunks of memory. Now memory are dumped for a few iterations, where the
size of pipe buffers is restricted.