implement read of /proc/uptime

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
read: translate pids into the reader's pidns

When reading a cgroupfs 'tasks' or 'cgroup.procs' file, we must translate
all pids from our namespace into the reader's.

TODO - convert writes

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Merge branch 'readtasks.pass.creds.2' into m.2